Semiconductor memory device including an SOI substrate

ABSTRACT

A semiconductor memory device includes a plurality of N and P channel MOS transistors. The plurality of MOS transistors are formed on an SOI (Silicon On Insulator) substrate. Each MOS transistor includes a source region, a drain region, and a body region located between the source region and the drain region. The body region of at least one N channel MOS transistor is electrically fixed. The body region of at least one P channel MOS transistor is rendered floating.

This application is a Divisional of application Ser. No. 09/499,368filed Feb. 7, 2000, which is a Divisional of application Ser. No.09/146,031 filed Sep. 2, 1998, now U.S. Pat. No. 6,091,647, which is aDivisional of application Ser. No. 08/876,755 filed Jun. 16, 1997, nowU.S. Pat. No. 5,825,696, which is a Continuation of application Ser. No.08/353,276 filed Dec. 5, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor memory devices, and moreparticularly to a dynamic random access memory (DRAM) formed on an SOI(Silicon On Insulator) substrate.

2. Description of the Background Art

A semiconductor memory device is typically divided into a volatilememory such as a RAM, and a non-volatile memory such as a ROM. Thevolatile memory is further divided into a DRAM and a static randomaccess memory (SRAM). The non-volatile memory includes a mask ROM, anEPROM, a flash memory, an EEPROM, a fuse ROM, and the like.

A DRAM has data stored by accumulating charge in the capacitor of amemory cell. Although such a DRAM requires a refresh operation, a DRAMhaving a large storage capacity can be manufactured at a low cost due toits simple structure of the memory cell.

Because data is stored by accumulating charge in a capacitor in a DRAM,the amount of charge stored in a capacitor is altered according to αparticles emitted from its package or interconnection material. Thischange in the amount of charge will result in data inversion, i.e., softerror.

The demand for DRAMs having a higher integration density is also great.The potential of mass production is appreciable for DRAMs having a largestorage capacity such as 256 M bits and 1 G bits. Although the gatelength is generally reduced to increase the integration density of aDRAM, this reduction in gate length has a limitation due to asignificant short channel effect as the channel length is reduced.

In recent years, large scaled integrated circuits (LSI) are developedhaving circuit elements such as transistors formed on an SOI substratewith an insulation layer buried in the semiconductor substrate.

FIG. 92 is a plan view showing a structure of a MOS transistor formed onan SOI substrate. FIGS. 93 and 94 are sectional views of the MOStransistor shown in FIG. 92 taken along lines 93—93 and 94—94,respectively.

Referring to FIGS. 92-94, an MOS transistor includes an n⁺ type sourceregion 1, an n⁺ type drain region 2, a p type body region 3, and a gateelectrode 4. Body region 3 is located between source region 1 and drainregion 2. When a predetermined potential is applied to gate electrode 4,a channel is formed in body region 3.

This MOS transistor is completely enclosed by a LOCOS oxide film 5 forisolation from an adjacent element. This MOS transistor is formed on anSOI substrate 6. SOI substrate 6 includes a silicon substrate 7, aburied oxide film 8 of SiO₂, and an SOI active layer 9. Source region 1,drain region 2, and body region 3 are formed in this SOI active layer 9.

Body region 3 attains a floating state electrically since it is enclosedby LOCOS oxide film 5 and isolated from silicon substrate 7 by buriedoxide layer 8. When body region 3 attains a floating state, thebreakdown voltage between the source and drain becomes as low asapproximately 3V due to a parasitic bipolar operation. There is also apossibility of a leakage current flow between the source and the drain.Furthermore, a body region 3 attaining a floating state induces thegeneration of a kink to disturb the drain current Id—drain voltage Vdcharacteristics. Therefore, the transistor cannot operate stably.

SUMMARY OF THE INVENTION

In view of the foregoing, a main object of the present invention is toprovide a semiconductor memory device formed on an SOI substrate.

Another object of the present invention is to provide a DRAM with almostno generation of a soft error.

A further object of the present invention is to provide a DRAM having agreater storage capacity.

Still another object of the present invention is to further increase thedata retaining time in a memory cell.

A still further object of the present invention is to improve thebreakdown voltage between the source and drain of a MOS transistor in asemiconductor memory device.

Yet a further object of the present invention is to reduce leakagecurrent between the source and drain of a MOS transistor in asemiconductor memory device.

Yet another object of the present invention is to operate a MOStransistor stably in a semiconductor memory device.

Yet a still further object of the present invention is to minimizeincrease in the layout area.

A semiconductor memory device according to an aspect of the presentinvention includes a plurality of N and P channel MOS semiconductorelements. The plurality of N and P channel MOS semiconductor elementsare formed on an SOI substrate. Each MOS semiconductor element includesa source region, a drain region, and a body region located between thesource and drain regions. At least one N channel MOS semiconductorelement of the plurality of N channel MOS semiconductor elements has itsbody region electrically fixed. At least one P channel MOS semiconductorelement of the plurality of P channel MOS semiconductor elements has itsbody region rendered floating electrically.

A semiconductor memory device according to another aspect of the presentinvention includes a plurality of N and P channel MOS semiconductorelements. The plurality of N and P channel MOS semiconductor elementsare formed on an SOI substrate. Each MOS semiconductor element includesa source region, a drain region, and a body region located between thesource and drain regions. Any body region of the plurality of N channelMOS semiconductor elements is fixed electrically. All the body regionsof the plurality of P channel MOS semiconductor elements are renderedfloating electrically.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of N and P channel MOSsemiconductor elements. The plurality of N and P channel MOSsemiconductor elements are formed on an SOI substrate. Each MOSsemiconductor element includes a source region, a drain region and abody region located between the source and drain regions. All the bodyregions of the plurality of N channel MOS semiconductor devices arefixed electrically. All the body regions of the plurality of P channelMOS semiconductor elements are rendered floating.

A semiconductor memory device according to still another aspect of thepresent invention includes a plurality of MOS capacitors. The pluralityof MOS capacitors are formed on an SOI substrate. Each MOS capacitorincludes a source region, a drain region connected to the source region,and a body region located between the source and drain regions. At leastone MOS capacitor of the plurality of MOS capacitors has its body regionconnected to its own source region.

A semiconductor memory device according to still a further aspect of thepresent invention includes a plurality of MOS transistors and aplurality of bit line pairs for storing data. The stored data is readout via a bit line pair. The plurality of MOS transistors and theplurality of bit line pairs are formed on an SOI substrate. Each MOStransistor includes a source region, a drain region, and a body regionlocated therebetween. The body region of a MOS transistor out of theplurality of MOS transistors having a source region or a drain regionconnected to any of the plurality of bit line pairs is electricallyfixed.

A semiconductor memory device according to yet a further aspect of thepresent invention includes a plurality of MOS transistors. The pluralityof MOS transistors are formed on an SOI substrate. Each MOS transistorincludes a source region, a drain region, and a body region locatedtherebetween. A variable potential is supplied to the body region of atleast one of the plurality of MOS transistors. This variable potentialis the reverse voltage with respect to the PN junction between one ofthe source and drain regions and the body region. Preferably, the bodyregion of the at least one MOS transistor is connected to its own sourceregion.

A semiconductor memory device according to yet another aspect of thepresent invention includes a plurality of bit line pairs, and aplurality of sense amplifiers. The plurality of sense amplifiers areprovided corresponding to the plurality of bit line pairs. Each senseamplifier amplifies the potential difference between a corresponding bitline pair. The plurality of bit line pairs and the plurality of senseamplifiers are formed on an SOI substrate. Each sense amplifier includesfirst and second N channel MOS transistors connected in series betweenthe corresponding bit line pair. The body region of the first N channelMOS transistor located between the source region and the drain region isconnected to its own source region. The body region of the second Nchannel MOS transistor located between the source region and the drainregion is connected to its own source region.

Preferably, each sense amplifier further includes first and second Pchannel MOS transistors connected in series between a corresponding bitline pair. The body region of the first P channel MOS transistor locatedbetween the source region and the drain region is connected to its ownsource region. The body region of the second P channel MOS transistorlocated between the source region and the drain region is connected toits own source region.

A semiconductor memory device according to yet a still further aspect ofthe present invention includes a plurality of MOS transistors and outputterminals for storing data. The stored data is externally output via theoutput terminal. The plurality of MOS transistors are formed on an SOIsubstrate. Each MOS transistor includes a source region, a drain region,and a body region located therebetween. Out of the plurality of the MOStransistors, the body region of the MOS transistor having the sourceregion connected to the output terminal is connected to its own sourceregion.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors. Apredetermined power supply voltage is supplied to the semiconductormemory device. The plurality of MOS transistors are formed on an SIOsubstrate. Out of the plurality of the MOS transistors, the body regionof the MOS transistor having a voltage higher than the power supplyvoltage supplied between the source region and the drain region iselectrically fixed.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors. The pluralityof MOS transistors are formed on an SOI substrate. Out of the pluralityof MOS transistors, the body region located between the source regionand the drain region of a MOS transistor carrying out an analogoperation is electrically fixed. Preferably, the MOS transistor thatcarries out an analog operation is a MOS transistor in a circuit thatprocesses a signal of an amplitude smaller than that of power supplyvoltage supplied to the semiconductor memory device.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors andinput/output lines for storing data. The stored data is read/written viathe input/output line. The plurality of MOS transistors and theinput/output lines are formed on an SOI substrate. Each MOS transistorincludes a source region, a drain region, and a body region locatedtherebetween. Out of the plurality of MOS transistors, the body regionof a MOS transistor having the source region or the drain regionconnected to the input/output line is electrically fixed.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors. The pluralityof MOS transistors are formed on an SOI substrate. Out of the pluralityof MOS transistors, the body region located between the source and drainregion of the MOS transistor in the input stage receiving an externallyapplied signal is electrically fixed.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors. The pluralityof MOS transistors are formed on an SOI substrate. Out of the pluralityof MOS transistors, the body region located between the source and drainregions of the MOS transistor at an output stage for outputting a signalis electrically fixed.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors. Any N channelMOS transistors out of the plurality of MOS transistors are connected inseries between an output node for providing a signal and a ground node.The plurality of MOS transistors are formed on an SOI substrate. Out ofthe any of the N channel MOS transistors, the body region locatedbetween the source region and the drain region of at least one N channelMOS transistors that does not have a source region directly connected tothe ground node is electrically fixed.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS transistors. The pluralityof MOS transistors are formed on an SOI substrate. Out of the pluralityof MOS transistors, the body region located between the source regionand the drain region of a MOS transistor having a gate length shorterthan a predetermined gate length is electrically fixed. Out of theplurality of MOS transistors, the body region located between the sourceregion and the drain region of a MOS transistor having a gate lengthlonger than the predetermined gate length is rendered floatingelectrically.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of first and second conductivechannel type MOS transistors. The plurality of first and secondconductive channel type MOS transistors are formed on an SOI substrate.At least one first conductive channel type MOS transistor of theplurality of first conductive channel type MOS transistors has a firstthreshold voltage. At least one first conductive channel MOS transistorof the plurality of first conductive channel MOS transistors has asecond threshold voltage differing from the first threshold voltage.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of first and second conductivechannel type MOS transistors. The plurality of first and secondconductive channel type MOS transistors are formed on an SOI substrate.A second conductivity type body region located between a firstconductivity type source region and a first conductivity type drainregion of at least one first conductive channel MOS transistor of theplurality of first conductive channel type MOS transistors includes aconductive layer having a first impurity concentration on the surfacethereof. A second conductivity type body region located between thefirst conductivity type source region and the first conductivity typedrain region between at least another first conductive channel type MOStransistor of the plurality of first conductive channel type MOStransistors includes a conductive layer having a second impurityconcentration differing from the first impurity concentration at thesurface thereof.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of first and second conductivechannel type MOS transistors. The plurality of first and secondconductive channel type MOS transistors are formed on an SOI substrate.A second conductivity type body region located between a firstconductivity type source region and the first conductivity type drainregion of at least one first conductive channel type MOS transistor outof the plurality of first conductive channel type MOS transistorsreceives a first potential. A second conductivity type body regionlocated between the first conductivity type source region and the firstconductivity type drain region of at least another first conductivechannel type MOS transistor out of the plurality of first conductivechannel type MOS transistors receives a second potential differing fromthe first potential.

A semiconductor memory device according to a further aspect of thepresent invention includes a memory cell array of a plurality of firstMOS transistors, and a peripheral circuit of a plurality of second MOStransistors. The plurality of first and second MOS transistors areformed on an SOI substrate. The plurality of first MOS transistors havea threshold voltage higher than that of the plurality of second MOStransistors.

A semiconductor memory device according to a further aspect of thepresent invention includes a plurality of MOS semiconductor elements.The plurality of MOS semiconductor elements are formed on an SOIsubstrate. The source and drain regions of any MOS semiconductorelements out of the plurality of MOS semiconductor elements are broughtinto contact with an insulation layer in the SOI substrate.

A semiconductor memory device according to a further aspect of thepresent invention includes a memory cell array of a plurality of firstMOS transistors, and a peripheral circuit of a plurality of second MOStransistors. The memory cell array and the peripheral circuit are formedon an SOI substrate. The source and drain regions of the plurality offirst MOS transistors are brought into contact with an insulation layerof the SOI substrate.

A semiconductor memory device according to a further aspect of thepresent invention includes at least one first semiconductor element andat least one second semiconductor element. An element isolation film forisolating the first and second semiconductor elements are formed on anSOI substrate. The element isolation film is brought into contact withan insulation layer in the SOI substrate.

A semiconductor memory device according to a further aspect of thepresent invention is formed on an SOI substrate. The SOI substrateincludes a semiconductor substrate, a buried insulation layer formed onthe semiconductor substrate, and a semiconductor active layer formed onthe buried insulation layer. The semiconductor memory device furtherincludes a supply circuit. The supply circuit supplies a predeterminedsubstrate potential to the semiconductor substrate of the SOI substrate.

Because the above-described semiconductor memory device in which allsemiconductor elements are formed on an SOI substrate has the bodyregion of at least one N channel MOS semiconductor element electricallyfixed, leakage current between the source and drain is reduced and thebreakdown voltage between the source and drain is increased. Becausethere is almost no kinks in the fixed body region, a stable Id-Vdcharacteristic can be obtained. Furthermore, because the body region ofat least one P channel MOS semiconductor element is rendered floatingelectrically, wiring for fixing the body region is not required, andincrease of the layout area is minimized. In general, the breakdownvoltage between the source and drain in an N channel MOS semiconductorelement is smaller than that of the P channel MOS semiconductor element.Here, the body region of an N channel MOS transistor is fixed, so thatthe breakdown voltage between the source and drain thereof is similar tothat of a P channel MOS semiconductor element.

Because the body region of a MOS capacitor is connected to its ownsource region, the body region thereof is fixed. Therefore, this MOScapacitor can operate stably. Furthermore, because the body region isconnected to the source region, wiring for supplying potential to thebody region is not required. Thus, there is almost no increase in thelayout area.

Because the body region of the MOS transistor connected to the bit linepair is fixed, leakage current flowing from the bit line pair via theMOS transistor, or the leakage current flowing to the bit line pair viathe MOS transistor is reduced.

Because a variable potential is applied to the body region of a MOStransistor that becomes a reverse voltage with respect to the PNjunction formed of the body region and the source/drain region, thetransistor does not carry out bipolar operation, and body effect doesnot occur. Therefore, this MOS transistor operates stably.

Because the body region of the MOS transistor to which high voltage isapplied between the source and drain is fixed, the breakdown voltagebetween the source and drain is increased, so that this transistor willoperate properly even when high voltage is applied between the sourceand drain.

Because the body region of a MOS transistor carrying out an analogoperation is fixed, there is almost no kinks in that transistor.Therefore, this transistor always operates stably.

Because the body region of a MOS transistor having source/drain regionsconnected to an input/output line is fixed, a great leakage current willnot flow between the source and drain, so that accurate data can beinput and output.

Because the body region of a MOS transistor at an input stage is fixed,a great leakage current will not flow between the source and drain, sothat a desired input impedance can be obtained.

Because the body region of a MOS transistor at an output stage is fixed,a great amount of leakage current will not flow between the source anddrain, so that a desired output impedance can be obtained.

Because the body region of an N channel MOS transistor that is notdirectly connected to a ground node is fixed, the threshold voltage ofthe transistor including that fixed body region is reduced, whereby thetransistor operates more speedily. Therefore, those transistor canoperate properly even when the power supply voltage is low.

Because the body region of a MOS transistor having a short gate lengthis fixed, the breakdown voltage between the source and drain of thattransistor is equal to that of a transistor having a greater gatelength. Also, the level of the leakage current flowing between thesource and drain of the transistor of the short gate length is similarto that of the transistor of the long gate length. Furthermore, becausethe body region of the MOS transistor of the long gate length isrendered floating, wiring for providing potential to the body region isnot required, so that increase in the layout area can be suppressed to aminimum.

Because transistors of the same conductivity type have more than onetype of threshold voltage, these transistors operate stably.

The junction capacitance of the source/drain region is reduced since asemiconductor element is formed in the thin SOI active layer.

Because an element isolation film such as a Locos oxide film is formedin a thin SOI active layer, the element isolation film comes intocontact with the insulation layer of the SOI substrate.

Because a predetermined substrate potential is supplied to thesemiconductor substrate of an SOI substrate, the semiconductor substrateis electrically fixed. Therefore, the potential of the semiconductorsubstrate will not change, so that change in the potential of thesemiconductor active layer will also not occur. As a result,semiconductor elements such as a transistor formed on the semiconductoractive layer operates stably.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing partially a structure of a memorycell, a sense amplifier, and an input/output circuit in a DRAM accordingto embodiment 1 of the present invention.

FIG. 2 is a block diagram showing the entire structure of a DRAM.

FIG. 3 is a timing chart showing an operation of the DRAM of FIG. 1.

FIG. 4 is a timing chart showing another operation of the DRAM of FIG.1.

FIG. 5 is a plan view showing a structure of the sense amplifier and theprecharge circuit of FIG. 1.

FIG. 6 is a sectional view of the sense amplifier of FIG. 1 taken alongline 6—6.

FIG. 7 is a plan view showing a general structure of a transistor in thebit line select circuit, the column select circuit or the memory cellshown in FIG. 1.

FIGS. 8 and 9 are plan views showing the structure of a sense amplifierand a precharge circuit of a DRAM according to embodiment 2 andembodiment 3, respectively, of the present invention.

FIGS. 10-12 are circuit diagrams showing a partial structure of a memorycell, a sense amplifier, and an input/output circuit of a DRAM accordingto embodiment 4, embodiment 5, and embodiment 6, respectively, of thepresent invention.

FIG. 13 is a plan view showing the structure of the sense amplifier andthe precharge circuit shown in FIG. 12.

FIGS. 14 and 15 are plan views showing a structure of the senseamplifier and the precharge circuit in a DRAM according to embodiment 7and embodiment 8, respectively, of the present invention.

FIGS. 16-19 are circuit diagrams showing a partial structure of a memorycell, a sense amplifier, and an input/output circuit of a DRAM accordingto embodiment 9, embodiment 10, embodiment 11, and embodiment 12,respectively, of the present invention.

FIGS. 20 and 21 are circuit diagrams showing structure of a word linedriving circuit of a DRAM according to embodiment 13 and embodiment 14,respectively, of the present invention.

FIG. 22 is a plan view showing a structure of an N channel MOStransistor in the word line driving circuit of FIG. 21.

FIGS. 23 and 24 are circuit diagrams showing a structure of boost signalpredecode circuit of a DRAM according to embodiment 15 and embodiment16, respectively, of the present invention.

FIG. 25 is a plan view showing a structure of an N channel MOS capacitorin a DRAM according to embodiment 17 of the present invention.

FIG. 26 is a plan view showing a structure of a P channel MOS capacitorof a DRAM according to embodiment 18 of the present invention.

FIG. 27 is a plan view showing a structure of an N channel MOS capacitorof a DRAM according to embodiment 19 of the present invention.

FIG. 28 is a plan view showing a structure of a P channel MOS capacitorof a DRAM according to embodiment 20 of the present invention.

FIG. 29 is a circuit diagram showing the entire structure of a boostedpower supply generation circuit of a DRAM according to embodiment 21 ofthe present invention.

FIG. 30 is a circuit diagram showing a structure of an outputpreamplifier and a write circuit of a DRAM according to embodiment 22 ofthe present invention.

FIG. 31 is a circuit diagram showing a structure of an input/output lineprecharge circuit and an input/output line equalize circuit of a DRAMaccording to embodiment 23 of the present invention.

FIG. 32 is a circuit diagram showing a structure of a row address bufferof a DRAM according to embodiment 24 of the present invention.

FIGS. 33-36 are circuit diagrams of a structure of a column addressbuffer of a DRAM according to embodiments 25-28, respectively, of thepresent invention.

FIGS. 37-40 are circuit diagrams of a structure of a clock input bufferof a DRAM according to embodiments 29-32, respectively, of the presentinvention.

FIG. 41 is a circuit diagram showing a structure of the sense amplifierdriving circuit of a DRAM according to embodiment 33 of the presentinvention.

FIG. 42 is a timing chart showing an operation of the sense amplifierdriving circuit of FIG. 41.

FIG. 43 is a circuit diagram showing a structure of a CAT circuit of aDRAM according to embodiment 34 of the present invention.

FIG. 44 is a timing chart showing an operation of the CAT circuit ofFIG. 43.

FIGS. 45 and 46 are circuit diagrams showing a structure of an N—Nbuffer of a DRAM according to embodiment 35 and embodiment 36,respectively, of the present invention.

FIG. 47 is a circuit diagram showing a structure of a NAND circuit of aDRAM according to embodiment 37 of the present invention.

FIG. 48 is a plan view showing a partial structure of the NAND circuitof FIG. 47.

FIGS. 49-52 are plan views showing a partial structure of the NANDcircuit of a DRAM according to embodiments 38-41, respectively, of thepresent invention.

FIG. 53 is a plan view showing a partial structure of the NAND circuitof FIG. 52.

FIGS. 54-59 are circuit diagrams showing a structure of a NAND circuitof a DRAM according to embodiments 42-47, respectively, of the presentinvention.

FIG. 60 is a sectional view of a memory cell in a DRAM according toembodiment 48 of the present invention.

FIG. 61 is a sectional view of the memory cell of FIG. 60 taken alongthe direction of a word line.

FIG. 62 is a sectional view showing a structure of a memory cell in aDRAM according to embodiment 49 of the present invention.

FIG. 63 is a sectional view of the memory cell of FIG. 62 taken alongthe direction of a word line.

FIGS. 64 and 65 are sectional views of a memory cell of a DRAM accordingto embodiment 50 and embodiment 51, respectively, of the presentinvention.

FIGS. 66-69 are layout diagrams showing the entire structure of a DRAMaccording to embodiments 52-55, respectively, of the present invention.

FIGS. 70-76 are diagrams showing the concept of a DRAM according toembodiments 56-62, respectively, of the present invention.

FIG. 77 is a sectional view of two P channel MOS transistors in a DRAMshown in FIG. 76.

FIG. 78 is a diagram showing the concept of a DRAM according toembodiment 63 of the present invention.

FIG. 79 is a sectional view of a sense amplifier of a DRAM according toembodiment 64 of the present invention.

FIG. 80 is a sectional view of a memory cell of a DRAM of FIG. 79.

FIG. 81 is a sectional view of a memory cell of FIG. 80 taken along thedirection of a word line.

FIG. 82 is a sectional view showing a structure of a memory cell of aDRAM according to embodiment 65 of the present invention.

FIG. 83 is a diagram showing the concept of a partial structure of aDRAM according to embodiment 66 of the present invention.

FIG. 84 is a sectional view showing a partial structure of a DRAMaccording to embodiment 67 of the present invention.

FIGS. 85-87 are perspective views showing a structure of a DRAMaccording to embodiments 68-70, respectively, of the present invention.

FIGS. 88 and 89 are diagrams of the concept of a partial structure of aDRAM according to embodiments 71 and 72, respectively, of the presentinvention.

FIG. 90 is a circuit diagram showing a partial structure of a memorycell, a sense amplifier, and an input/output circuit of a DRAM accordingto embodiment 73 of the present invention.

FIG. 91 is a timing chart showing an operation of the DRAM of FIG. 90.

FIG. 92 is a plan view showing a structure of a conventional N channelMOS transistor formed on an SOI substrate.

FIGS. 93 and 94 are sectional views of the transistor of FIG. 92 takenalong lines 93—93, and 94—94, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described hereinafterwith reference to the drawings. In the drawings, the same referencecharacters denote the same or corresponding components.

Embodiment 1

FIG. 2 is a block diagram showing an entire structure of a DRAMaccording to embodiment 1 of the present invention. Referring to FIG. 2,a DRAM 10 includes a memory cell array 11, a row decoder 12, a columndecoder 13, a sense amplifier group 14, an input/output circuit 15, arow-and-column address buffer 16, an input buffer 17, an output buffer18, and a clock generation circuit 19.

In memory cell array 11, a plurality of word lines (not shown) aredisposed along the row direction, and a plurality of bit line pairs (notshown) are disposed along the column direction, with a plurality ofmemory cells (not shown) at the crossings thereof. Row decoder 12responds to a row address signal from address buffer 16 to select anddrive one of the plurality of word lines. Column decoder 13 responds toa column address signal from address buffer 16 to select one of theplurality of bit line pairs. Sense amplifier group 14 includes aplurality of sense amplifiers. The plurality of sense amplifiers areprovided corresponding to the plurality of bit line pairs. Each senseamplifier amplifies the potential difference of a corresponding bit linepair. Input/output circuit 15 provides the potential of the bit linepair selected by column decoder 13 to output buffer 18. Output buffer 18amplifies the provided potential to output the same as output dataDQ₁-DQ₄. Input buffer 17 amplifies externally applied input dataDQ₁-DQ₄. Input/output circuit 15 provides the input data amplified ininput buffer 17 to the bit line pair selected by column decoder 13.Address buffer 16 selectively provides externally applied addresssignals A0-A11 to row decoder 12 and column decoder 13.

FIG. 1 is a circuit diagram showing in detail memory cell array 11,sense amplifier group 14 and input/output circuit 15 of FIG. 12partially. Referring to FIG. 1, word lines WL1, WL2, . . . and bit linepairs BL0, /BL0, BL1, /BL1 crossing these word lines are disposed inmemory cell array 11. A memory cell 27 is disposed at the crossingbetween bit lines BL0, /BL0 and word lines WL1, WL2.

One sense amplifier 20 is disposed corresponding to two bit line pairsBL0, /BL0 and BL1, /BL1. Bit lines BL0 and /BL0 are connected to senseamplifier 20 via a bit line select circuit 26A. Bit lines BL1 and /BL1are connected to sense amplifier 20 via a bit line select circuit 26B.Bit line select circuit 26A responds to a bit line select signal BLI0 toconnect bit line pair BL0, /BL0 to sense amplifier 20, whereby senseamplifier 20 amplifies the potential difference between bit lines BL0and /BL0. Bit line select circuit 26B responds to a bit line selectsignal BLI1 to connect bit lines BL1 and /BL1 to sense amplifier 20,whereby sense amplifier 20 amplifies the potential difference betweenbit lines BL1 and /BL1. Thus, embodiment 1 employs the so-called sharedsense amplifier system.

One bit line precharge circuit 23 is provided corresponding to senseamplifier 20. Precharge circuit 23 corresponds to a bit line equalizesignal BLEQ to precharge bit line pairs BL0, /BL0, BL1, /BL1 to apredetermined potential V_(BL).

Also, one column select circuit 29 is provided corresponding to one or aplurality of sense amplifiers 20. Column select circuit 29 responds to acolumn select signal CSL to connect bit line pairs BL0, /BL0, BL1, /BL1to an input/output line pair IO and /IO.

A drive line precharge circuit 22 is provided between sense amplifierdrive lines 21A and 21B to drive sense amplifier 20. Precharge circuit22 responds to an equalize signal BLEQ to precharge sense amplifierdrive lines 21A and 21B to a predetermined potential V_(BL). Senseamplifier drive line 21A is also connected to a ground node via an Nchannel MOS transistor Qs1 which becomes conductive in response tocontrol signal SOF. Sense amplifier drive line 21A is also connected toa ground node via an N channel MOS transistor Qs2 which becomesconductive in response to control signal SON. Sense amplifier drive line21B is connected to a power supply node via a P channel MOS transistorQs3 which becomes conductive in response to a control signal SOP.

Dummy word lines DWL1 and DWL2 are disposed parallel to word lines WL1and WL2. A dummy cell 28 is disposed at the respective crossings betweendummy word lines DWL1, DWL2 and bit lines BL0, /BL0. When word lines WL1and WL2 rise, dummy cell 28 cancels the noise generated in bit lines BL0and /BL0.

Sense amplifier 20 includes N channel MOS transistors Qn1 and Qn2connected in series between a bit line pair, and P channel MOStransistors Qp1 and Qp2 connected in series between a bit line pair.Transistors Qn1 and Qp1 have their gate electrodes connected to bitlines /BL0 and /BL1, and transistors Qn2 and Qp2 have their gateelectrodes connected to bit lines BL0 and BL1. Transistors Qn1 and Qn2have their source electrodes connected to sense amplifier driven line 21a, and transistors Qp1 and Qp2 have their source electrodes connected tosense amplifier drive line 21 b.

Each memory cell 27 includes an N channel MOS transistor Qm that servesas a transfer gate, and a capacitor Cm for storing data. Transistor Qmhas its gate electrode connected to a corresponding word line WL1 orWL2, and one of the source/drain electrodes connected to a correspondingbit line BL0 or /BL0. Capacitor Cm has one electrode connected to theother source/drain electrode of transistor Qm. Cell plate potential Vcpis supplied to the other electrode of capacitor Cm.

Similar to memory cell 27, each dummy cell 28 includes an N channel MOStransistor Qd and a capacitor Cd. Transistor Qd has its gate electrodeconnected to a corresponding dummy word line DWL1 or DWL2, and onesource/drain electrode connected to a corresponding bit line BL0 or/BL0. Capacitor Cd has one electrode connected to the other source/drainelectrode. Cell plate potential Vcp is supplied to the other electrodeof capacitor Cd.

Bit line select circuit 26A includes two N channel MOS transistors Qbwhich become conductive in response to bit line select signal BLI0. Bitline select circuit 26B includes two N channel MOS transistors Qb thatbecome conductive in response to bit line select signal BLI1.

Bit line precharge circuit 23 includes an N channel MOS transistor Qeconnected between a bit line pair, and two N channel MOS transistors Qpcconnected in series between a bit line pair. Transistors Qe and Qpc havetheir gate electrodes connected to an equalize line 24. Transistors Qpchave their source electrodes connected to precharge line 25.

Column select circuit 29 includes an N channel MOS transistor Qioconnected between bit lines BL0, BL1 and input/output line IO, and whichbecomes conductive in response to column select signal CSL, and an Nchannel MOS transistor Qio connected between bit lines /BL0, /BL1 andinput/output line /IO, and which becomes conductive in response tocolumn select signal CSL.

A drive dedicated precharge circuit 22 includes an N channel MOStransistor Qse connected between drive lines 21A and 21B, and two Nchannel MOS transistors Qsp connected in series between drive lines 21Aand 21B. Transistors Qse and Qsp have their gate electrodes connected toequalize line 24. Transistors Qsp have their source electrodes connectedto precharge line 25.

The operation of the circuitry shown in FIG. 1 will be described withreference to the timing chart of FIG. 3.

Referring to FIG. 3(a), address signals A0-A11 are strobed in responseto a fall of external row address strobe signal /RAS. When bit line pairBL0, /BL0 is selected according to that address signal, bit line selectsignal BLI0 rises as shown in FIG. 3(d). Bit line select signal BLI1 ismaintained at a L level (logical low). Therefore, bit line pair BL0,/BL0 is connected to sense amplifier 20.

Since bit line equalize signal BLEQ attains a H level (logical high) asshown in FIG. 3(f), transistors Qpc of bit line precharge circuit 23 areboth conductive. Therefore, precharge potential Vb1 is applied to bitline pair BL0, /BL0. Because transistor Qe of bit line precharge circuit23 is also conductive, the potentials of bit lines BL0 and /BL0 areequal to each other. The potential of bit line pair BL0, /BL0 takes anintermediate level between a H level and a L level as shown in FIG. 3(j)because a potential V_(CC)/2 which is half the power supply potential issupplied as precharge potential Vb1.

This equalize signal BLEQ of a H level is also applied to the gateelectrodes of transistors Qse and Qsp of drive dedicated prechargecircuit 22, so that sense amplifier drive lines 21A and 21B areprecharged to potential V_(CC)/2 which is half the power supplypotential, similar to bit line pair BL0, /BL0.

Then, when word line WL1 rises as shown in FIG. 3(b), transistor Qd of acorresponding memory cell 27 is rendered conductive, whereby the chargein capacitor Cm is read out on bit line BL0. When data of a L level isstored in memory cell 27, the potential of bit line BL0 becomes slightlylower than precharge potential Vb1 as shown in FIG. 3(j). As a result, apotential difference is generated between bit lines BL0 and /BL0.

Then, when control signal SOF rises as shown in FIG. 3(g), transistorQs1 is rendered conductive, whereby the charge of sense amplifier driveline 21A flows to the ground node via transistor Qs1. This causespotential SAN of sense amplifier drive line 21A to decrease towardsground potential V_(SS).

Then, when control signal SON rises as shown in FIG. 3(h), transistorQs2 is rendered conductive, whereby the charge of sense amplifier driveline 21A flows to the ground node via transistor Qs2. As a result,potential SAN of sense amplifier drive line 21A further decreasestowards ground potential V_(SS).

When control signal SOP falls as shown in FIG. 3(i), transistor Qs3 isrendered conductive, whereby charge is supplied from the power supplynode to sense amplifier drive line 21B via transistor Qs3. As a result,potential SAB of sense amplifier drive line 21B gradually increasestowards power supply potential V_(CC).

Because sense amplifier drive signal SAN gradually decreases towardsground potential V_(SS) and sense amplifier drive signal SAP graduallyincreases towards power supply potential V_(CC) as described above,sense amplifier 20 reduces the potential of bit line BL0 to a L level,and the potential of bit line /BL0 to a H level, as shown in FIG. 3(j).Therefore, sense amplifier 20 latches complementary data correspondingto the data of memory cell 27.

Then, when column select signal CSL rises, transistors Qio of columnselect circuit 29 both attain a conductive state. This causes thepotential of bit line BL0 to be provided to input/output line IO viatransistor Qio and the potential of bit line /BL0 to input/output line/IO via transistor Qio. The potentials at input/output lines IO and /IOare amplified by output buffer 18 to be output as output data.

Although the case where a bit line pair is precharged to V_(CC)/2 isdescribed here, the bit line pair may be precharged to V_(CC). In thiscase, the capacitance of capacitor Cd in dummy cell 28 must be differentfrom that of capacitor Cm in memory cell 27. For example, thecapacitance of capacitor Cd may be half that of capacitor Cm.

FIG. 4 is a timing chart of a bit line pair precharged to V_(CC). Whenbit line equalize signal BLEQ attains a H level as shown in FIG. 4(f),the bit line pair is precharged to a H level, i.e. to power supplypotential V_(CC), as shown in FIG. 4(j). By pulling up DWL2simultaneously to the rise of word line WL1, a potential difference isgenerated between the bit line pair.

When the power supply level is established in a hierarchical manner, aninternal power supply potential is generated by down-converting externalpower supply potential, and an internal ground potential is generated byboosting the external ground potential. In this case, sense amplifierdrive signal SAN gradually decreases from precharge potential towardsinternal ground potential which is higher than external groundpotential, and sense amplifier drive signal SAP gradually increasestowards internal power supply potential which is lower than the externalpower supply potential. Therefore, sense amplifier 20 raises thepotential of one sense amplifier to the level of internal power supplypotential, and the potential of the other bit line to the level ofinternal power supply potential.

In sense amplifier 20 of embodiment 1, a constant ground potentialV_(SS) is applied to the body regions of N channel MOS transistors Qn1and Qn2, whereby the body regions are electrically fixed. Also, aconstant power supply potential V_(CC) is applied to the body regions ofP channel MOS transistors Qp1 and Qp2, whereby the body regions areelectrically fixed.

Therefore, a kink will not be generated in these transistors Qn1, Qn2,Qp1, and Qp2, so that a stable Id-Vd characteristic is obtained. Thus,sense amplifier 20 carries out a stable analog operation.

Because the body regions of transistors Qn1, Qn2, Qp1 and Qp2 are fixed,the leakage current between the source and drain is reduced. Therefore,the charge of bit lines BL0, /BL0, BL1 and /BL1 will not leak viatransistors Qn1, Qn2, Qp1 and Qp2. Thus, the potential differencegenerated between a bit line pair when data is read out from memory cell27 can be maintained at a sufficient high level.

In memory cell 27 of embodiment 1, a constant ground potential V_(SS) isapplied to the body region of an N channel MOS transistor, so that thebody region is electrically fixed. Therefore, the subthresholdcharacteristic is improved, and the leakage current approximates thephysical limit value. Thus, charge that leaks from capacitor Cm viatransistor Qm is determined by the leakage at the PN junction. In atransistor formed on a thin film SOI, there is at least no PN junctionplane that is parallel with an SOI substrate. The leakage current at aPN junction is proportional to the surface area of the PN junction, sothat data retaining time period is increased. In dummy memory cell 28, aconstant ground potential V_(SS) is applied to the body region of Nchannel MOS transistor Qd, so that the body region is electricallyfixed, similar to memory cell 27.

In bit line precharge circuit 23 of embodiment 1, constant groundpotential V_(SS) is applied to the body regions of N channel MOStransistors Qe and Qpc, so that the body regions are electrically fixed.Therefore, the charge in the bit line will not leak via thesetransistors Qe and Qpc. This prevents the read out potential differencegenerated between bit lines from being reduced, so that the potentialdifference is reliably amplified by a sense amplifier.

In drive dedicated precharge circuit 22, a constant ground potentialV_(SS) is applied to the body regions of transistors Qse and Qsp, sothat the body regions are electrically fixed, similar to bit lineprecharge circuit 23. Also, constant ground potential V_(SS) is appliedto the body regions of N channel MOS transistors Qs1 and Qs2, so thatthe body regions are electrically fixed. Power supply potential V_(CC)is applied to the body region of P channel MOS transistor Qs3, so thatthe body region is electrically fixed.

In bit line select circuits 26A and 26B in embodiment 1, a constantground potential V_(SS) is applied to the body region of N channel MOStransistor Qp, so that the body region is electrically fixed. The chargein the bit lines do not leak via transistor Qb, so that the read outpotential difference is maintained sufficiently.

In column select circuit 29 of embodiment 1, a constant ground potentialV_(SS) is applied to the body region of N channel MOS transistor Qio, sothat the body region is electrically fixed. Therefore, the charge in thebit lines will not leak via transistor Qio, so that the read outpotential difference is maintained sufficiently. Thus, correct data isread out to input/output lines IO and /IO via column select circuit 29.

FIG. 5 is a plan view of sense amplifier 20 partially and prechargecircuit 23 entirely shown in FIG. 1. FIG. 6 is a sectional view of senseamplifier 20 of FIG. 5 taken along line 6—6. In FIG. 5, only an Nchannel sense amplifier formed of N channel MOS transistors Qn1 and Qn2is shown.

Referring to FIGS. 5 and 6, an n⁺ type source region 1 of transistor Qn1is common to the source region of transistor Qn2. Source region 1 isconnected to sense amplifier drive line 21A to which a sense amplifierdrive signal SAN is provided via contact hole CH.

An n⁺ type drain region 2 of transistor Qn1 is connected to bit line BL1via a contact hole CH. n⁺ type drain region 2 of transistor Qn2 isconnected to bit line /BL1 via contact hole CH. Transistor Qn1 has itsgate electrode 4 connected to bit line /BH1 via contact hole CH.Transistor Qn2 has gate electrode 4 connected to bit line BL1 viacontact hole CH.

A p⁺ type contact region 31 is formed on a p type body region 3 oftransistor Qn1. Contact region 31 is connected to a body fix line 30Cvia an intermediate layer 32 such as of a polypad. Ground potentialV_(SS) is supplied to body fix line 30C. A constant ground potentialV_(SS) is supplied to body region 3. Contact region 31 is also formed inbody region 3 of transistor Qn2. Body region 3 of transistor Qn2 isconnected to body fix line 30B via contact region 31 and intermediatelayer 32. Ground potential V_(SS) is also supplied to body fix line 30B.Therefore, ground potential V_(SS) is also provided to body region 3 oftransistor Qn2.

Referring to FIG. 6, a first interlayer insulation film 33 is formed ongate electrode 4. Contact hole CH is formed at a predetermined positionin interlayer insulation film 33. Intermediate layer 32 is formed oncontact hole CH. A second interlayer insulation film 34 is formed onfirst interlayer insulation film 33 and intermediate layer 32. Contacthole CH is formed at a predetermined position of second interlayerinsulation film 34. Bit lines BL and /BL are formed on second interlayerinsulation film 34.

A third interlayer insulation film 35 is formed on second interlayerinsulation film 34 and bit lines BL and /BL. Contact hole CH is formedat a predetermined position of third interlayer insulation film 35.Contact hole CH is provided above intermediate layer 32. Body fix lines30B and 30C are formed on third interlayer insulation film 35. Body fixline 30C is formed above contact hole CH to form contact withintermediate layer 32. A fourth interlayer insulation film 36 is formedon third interlayer insulation film 35 and body fix lines 30B and 30C.

Transistor Qn1 of sense amplifier 20 is formed on an SOI substrate 6.Because a buried oxide layer 8 is provided at a shallow region in thisSOI substrate, the thickness of an SOI active layer 9 is small. As aresult, the bottom of LOCOS oxide film 5 reaches buried oxide layer 8.Source region 1 and drain region 2 of transistor Qn1 also reach buriedoxide layer 8. As a result, body fix line 30C is connected to bodyregion 3 of transistor Qn1 which is completely isolated from itsperiphery by LOCOS oxide film 5 and buried oxide film 8. Thereforeground potential V_(SS) is provided thereto.

In precharge circuit 23, equalize line 24 forms the gate electrodes ofall transistor Qe and Qpc. Therefore, p type body region 3 of thesetransistors Qe and Qpc are common to each other. A p⁺ type contactregion 31 is formed in body region 3. Contact region 31 is connected tobody fix line 30A via contact hole CH. Ground potential V_(SS) issupplied to body fix line 30A. Therefore, ground potential V_(SS) isapplied to body regions 4 of transistors Qe and Qpc.

Drive precharge circuit 22 has a structure similar to that of bit lineprecharge circuit 23.

FIG. 7 is a plan view showing a general structure of transistors Qm, Qd,Qb and Qio of memory cell 27, dummy cell 28, bit line select circuits26A and 26B, and column select circuit 29. Referring to FIG. 7, p⁺ typecontact region 31 is formed in p type body region 3 of thesetransistors. Ground potential V_(SS) is supplied to contact region 31.As a result, body region 3 is fixed electrically.

Because a DRAM according to embodiment 1 is formed on an SOI substrateso that a silicon substrate 7 is electrically isolated by SOI activelayer 9 and buried oxide layer 8, the charge generated in siliconsubstrate 7 due to α particles impinging thereto will not flow intosource region 1, drain region 2, and body region 3. Furthermore, thereis almost no generation of charges due to α particles in source region1, drain region 2, and body region 3 since the regions 1, 2 and 3 areextremely small. Thus, there is almost no generation of the so-calledsoft error.

Because the bottom of the source/drain region of transistor Qm formingmemory cell 27 also reaches buried oxide layer 8, there is no PNjunction plane parallel to SOI substrate 6. Only a PN junction planeperpendicular to SOI substrate 6 exists. The leakage current in a PNjunction is proportional to the surface area of the PN junction.Therefore, the charge leaking from capacitor Cm via the source/drainregions is reduced according to the surface area, and the retaining timeperiod of data is increased accordingly. Furthermore, because thejunction capacitance of the source/drain region is also reduced, theread out potential difference generated between the bit lines isincreased, and power consumption becomes lower.

The charge of the bit line will not leak via a transistor connected tothe bit line since its body region is fixed. Therefore, the read outpotential difference generated between the bit lines is maintainedsufficiently. Furthermore, there are almost no kinks in this transistorssince the body region of the transistor of sense amplifier 20 is alsofixed. Thus, sense amplifier 20 amplifies the read out potentialdifference stably.

Embodiment 2

FIG. 8 is a plan view showing a structure of sense amplifier 20partially and precharge circuit 23 entirely in a DRAM according toembodiment 2 of the present invention.

Referring to FIG. 8, embodiment 2 has both body regions 3 protruding inthe same direction with a contact region 31 formed at that protrudingportion. Both contact regions 31 are connected to one body fix line 30Cvia respective contact holes CH. Also, both gate electrodes 4 protrudein the same direction. The protruding portions thereof are connected tobit lines BL1 and /BL1 via respective contact holes CH.

Embodiment 2 has a layout area smaller than that of embodiment 1 sincebody fix line 30C for fixing body regions 3 of transistors Qn1 and Qn2are common.

Embodiment 3

FIG. 9 is a plan view showing a structure of sense amplifier 20partially and precharge circuit 23 entirely of a DRAM according toembodiment 3 of the present invention. Referring to FIG. 9, prechargecircuit 23 differs from the precharge circuit of embodiment 1 in that itis disposed in a 180° rotated position. Body regions 3 of transistors Qeand Qpc in precharge circuit 23 are connected to body fix line 30B viacontact region 31 and contact hole CH. Body region 3 of transistor Qn2of sense amplifier 20 is also connected to body fix line 30B via contactregion 31 and contact hole CH.

Embodiment 3 has a layout area smaller than that of embodiment 1 sincebody fix line 30B for fixing body region 3 of transistor Qn2 of senseamplifier 20 is common to the body fix line for fixing body region 3 oftransistors Qe and Qpc in precharge circuit 23.

Embodiment 4

FIG. 10 is a circuit diagram showing a memory cell array, a senseamplifier, and an input/output circuit partially of a DRAM according toembodiment 4. Referring to FIG. 10, embodiment 4 has a negativepotential V_(BB) supplied to the body region of transistor Qm of memorycell 27, different from the potential of embodiment 1. Negativepotential V_(BB) is also applied to the body region of transistor Qd indummy cell 28. Therefore, the N channel MOS transistors in embodiment 4have two types of threshold voltages.

Because only the threshold voltages of transistors Qm and Qd in memorycell 27 and dummy cell 28 are great, sub threshold current does noteasily flow in transistors Qm and Qd in embodiment 4. Therefore, thedynamic data retention of the bit line amplitude according to a senseoperation is further improved in a deselect memory cell. Therefore, thedata retaining time period of a memory cell is increased.

Embodiment 5

FIG. 11 is a circuit diagram showing a memory cell array, a senseamplifier, and an input/output circuit partially of a DRAM according toembodiment 5 of the present invention.

Referring to FIG. 11, embodiment 5 has negative potential V_(bb) appliedto the body regions of all N channel MOS transistors Qm, Qd, Qb, Qpc,Qe, Qn1, Qn2, and Qio. A negative potential Vbb may be applied to thebody region of all N channel MOS transistors as in embodiment 5.

Embodiment 6

FIG. 12 is a circuit diagram showing a memory cell array, a senseamplifier, and an input/output circuit partially of a DRAM according toembodiment 6 of the present invention. Referring to FIG. 12, embodiment6 has the body regions of four transistors Qn1, Qn2, Qp1, and Qp2 ofsense amplifier 20 connected to its own source electrode. Morespecifically, the body regions of transistors Qn1 and Qn2 are connectedto sense amplifier drive line 21A. The body regions of transistors Qp1and Qp2 are connected to sense amplifier drive line 21B. Therefore, avariable potential is applied to the body regions of transistors Qn1 andQn2 that gradually decreases towards ground potential V_(SS) fromprecharge potential V_(BL). Also, a variable potential is provided tothe body regions of transistors Qp1 and Qp2 that increases towards powersupply potential V_(CC) from precharge potential V_(BL). Therefore, theso-called body effect is not generated in transistors Qn1, Qn2, Qp1 andQp2 since the same voltage is always applied to the PN junction betweenthe body region and the source region. Therefore, the sensitivity ofsense amplifier 20 is improved in comparison with that of embodiment 1.Sense amplifier 20 can operate speedily even in the case where a lowpower supply voltage is supplied.

When a DRAM is formed on a silicon substrate 1, the sense amplifier mustbe completely isolated from the substrate and other wells in order toprovide synchronization between the substrate potential of thetransistor in the sense amplifier and the source potential as shown inFIG. 12. Therefore, a triple well structure is generally employed.Furthermore, in order to reduce leakage current by subthreshold in awriting operation, the well must be fixed at a constant potential. Thiscauses increase in power consumption since the charge in the junctioncapacitance of that well is charged/discharged. Furthermore, a regionfor fixing the well potential is required when a triple well structureis employed. This results in increase in the layout area.

In contrast, embodiment 6 has the bottom of the body region brought intocontact with the buried oxide layer, so that the junction capacitance isextremely small. Also, leakage current due to subthreshold will notincrease since the body region is fixed. Furthermore, the layout area issmall since it is not necessary to form a well or the like.

FIG. 13 is a plan view showing sense amplifier 20 partially andprecharge circuit 23 of FIG. 12. Referring to FIG. 13, a p⁺ type commonregion 38 is formed at a portion of source region 1. Body region 3 oftransistor Qn1 is connected to source region 1 via common region 38.Body region 3 of transistor Qn2 is connected to source region 1 viacommon region 38. Because sense amplifier drive signal SAN is providedto this source region 1 via contact hole CH, these body regions 3 areelectrically fixed during the application of forward voltage in the PNjunction between common region 38 and source region 1. Morespecifically, the potential of body region 3 is always higher than thepotential of source region 1 by the barrier potential of the PNjunction. Because body fix lines 30B and 30C do not have to be providedin the embodiment 6, the layout area is smaller than that of embodiment1.

Embodiment 7

FIG. 14 is a plan view showing a sense amplifier and a precharge circuitof a DRAM according to embodiment 7 of the present invention. Referringto FIG. 14, embodiment 7 has a p⁺ type common region 38 of a sizesubstantially equal to that of source region 1 formed between bodyregions 3. Source region 1 is connected to sense amplifier drive line21A via contact hole CH. Common region 38 is also connected to senseamplifier drive line 21A via contact hole CH. Body region 3 oftransistor Qn1 is connected to sense amplifier drive line 21A via commonregion 38 and contact hole CH. Body region 3 of transistor Qn2 isconnected to sense amplifier drive line 21A via common region 38 andcontact hole CH. Therefore, body regions 3 are always electrically fixedsince the potentials of common region 38 and source region 8 are alwaysthe same.

Embodiment 8

FIG. 15 is a plan view showing a sense amplifier and a precharge circuitof a DRAM according to embodiment 8. Referring to FIG. 15, a p⁺ typecommon region 38 is connected to either side of source region 1. Also,two contact hole CH are formed over source region 1 and common region38. Therefore, body region 3 of transistor Qn1 is connected to senseamplifier drive line 21A via two common regions 38 and two contact holesCH, and body region 3 of transistor Qn2 is connected to sense amplifierdrive line 21A via two common regions 38 and two contact holes CH. Inembodiment 8, the effective channel length of transistors Qn1 and Qn2 isincreased since a small common region 38 is formed at either side ofsource region 1. Body regions 3 of transistors Qn1 and Qn2 are reliablyfixed even when the effective channel length thereof is long by virtueof the provision of two common regions 38. The potential of body region3 promptly follows potential SAN of sense amplifier drive line 21A evenwhen located remote from common region 38. As a result, this senseamplifier operates more stably than that of embodiment 7 of FIG. 14.

Embodiment 9

FIG. 16 is a circuit diagram showing a memory cell array, a senseamplifier, and an input/output circuit partially of a DRAM according toembodiment 9. Referring to FIG. 16, embodiment 9 the body regions oftransistors Qb and Qio in bit line select circuits 26A and 26B andcolumn select circuit 29 are set to a floating state electrically. Agreat amount of leakage current will not flow in these transistors Qband Qio even when rendered to a floating state.

Embodiment 9 has the number of the body fix lines reduced in comparisonwith that of embodiment 6 of FIG. 12 since the body regions of several Nchannel MOS transistors are electrically fixed, and the body regions ofthe remaining n channel MOS transistors are rendered floating.Therefore, the region for a body fix line is reduced, resulting in asmaller layout area.

Embodiment 10

FIG. 17 is a circuit diagram showing a memory cell array, a senseamplifier, and an input/output circuit of a DRAM according to embodiment10 of the present invention. Referring to FIG. 17, embodiment 10 has thebody regions of all P channel MOS transistors set to a floating stateelectrically. More specifically, the body regions of transistors Qp1 andQp2 in sense amplifier 20 are rendered floating. The body regions of allN channel MOS transistors are fixed electrically.

Because the breakdown voltage between the source and drain of a Pchannel MOS transistor is generally higher than that of an N channel MOStransistor, the body region of an N channel MOS transistor should befixed. Embodiment 10 has a layout area smaller than that of embodiment 1since a body fix region and a body fix line for fixing the body regionof a P channel MOS transistor are not required.

Embodiment 11

FIG. 18 is a circuit diagram showing a structure of a memory cell array,a sense amplifier, and an input/output circuit of a DRAM according toembodiment 11 of the present invention. Referring to FIG. 18, embodiment11 has bit lines BL0 and /BL0 disposed at either side of sense amplifier20. In other words, the so-called open bit line structure is employed.

Similar to the above-described embodiment 1, in embodiment 11, groundpotential V_(SS) is applied to the body regions of N channel MOStransistors Qn1 and Qn2 in sense amplifier 20, and power supplypotential V_(CC) is applied to the body regions of P channel MOStransistors Qp1 and Qp2. Ground potential V_(SS) is applied to the bodyregions of N channel MOS transistors Qe and Qpc in bit line prechargecircuit 23. Also, ground potential V_(SS) is applied to the body regionsof N channel MOS transistors Qse and Qsp in sense amplifier drive lineprecharge circuit 22. Ground potential V_(SS) is applied to the bodyregion of N channel MOS transistor Qio in column select circuit. Groundpotential V_(SS) is also applied to the body region of N channel MOStransistor Qm of memory cell 27. Ground potential V_(SS) is also appliedto the body region of N channel MOS transistor Qd in dummy cell 28.

In addition to the advantages of embodiment 1, embodiment 11 can have amemory cell 27 disposed at all the crossings of a word line and a bitline since the open bit line structure is employed.

Embodiment 12

FIG. 19 is a circuit diagram showing a structure of a memory cell array,a sense amplifier, and an input/output circuit of a DRAM according toembodiment 12 of the present invention. Referring to FIG. 19, the bodyregions of four transistors Qn1, Qn2, Qp1, and Qp2 of sense amplifier 20are connected to its own source electrode. Therefore, sense amplifierdrive signal SAN is applied to the body regions of N channel MOStransistors Qn1 and Qn2. Sense amplifier drive signal SAP is applied tothe body regions of P channel MOS transistors Qp1 and Qp2.

In addition to the advantages of embodiment 6 of FIG. 12, embodiment 12can have memory cell 27 provided at all the crossings between a wordline and a bit line since an open bit line structure is employed.

Embodiment 13

FIG. 20 is a circuit diagram showing a row decoder partially of a DRAMaccording to embodiment 13 of the present invention. Referring to FIG.20, a word line drive circuit in a row decoder includes eight N channelMOS transistors Qr1-Qr8 at the final stage. Transistors Qr1-Qr2 areconnected in series, and the source electrode of transistor Qr1 isconnected to word line WL0. Transistors Qr3 and Qr4 are connected inseries, and the source electrode of transistor Qr3 is connected to wordline WL1. Transistors Qr5 and Qr6 are connected in series, and thesource electrode of transistor Qr5 is connected to word line WL2.Transistors Qr7 and Qr8 are connected in series, and the sourceelectrode of transistor Qr7 is connected to word line WL3.

This word line drive circuit is activated in response to signals Xj, Xkand Xl which are predecoded row address signals. When the word linedrive circuit is activated and one of boost signals RX0-RX3 is appliedto the drain electrode of a corresponding transistor, a correspondingone of word lines WL0-WL3 rises. Because boost signals RX0-RX3 are atpotentials higher than power supply potential V_(CC), word lines WL0-WL3rise to a potential higher than power supply potential V_(CC).Therefore, a voltage greater than the power supply voltage is appliedbetween the source and drain of transistors Qr1-Qr8.

When boost signals RX0-RX3 are applied when this word line drive circuitis not activated, the potential of the body regions of transistors Qr1,Qr3, Qr5 and Qr7 rise due to coupling of the parasitic capacitance,resulting in reduction of the threshold value. This causes leakage ofthe boosted potential via transistors Qr1, Qr3, Qr5 and Qr7, so that theboosted potential becomes insufficient. When the leaking boostedpotential is applied to a deselect word line, data will leak from thede-select memory cell.

In embodiment 13, ground potential V_(SS) is applied to the body regionsof transistors Qr1-Qr12, whereby the body regions thereof areelectrically fixed. This causes increase in the breakdown voltagebetween the source and drain of transistors Qr1-Qr12, so that this wordline drive circuit operates properly. This word line drive circuitoperates stably since reduction in the threshold value due to couplingof the parasitic capacitance is suppressed.

Embodiment 14

FIG. 21 is a circuit diagram showing a row decoder partially of a DRAMaccording to embodiment 14 of the present invention. Referring to FIG.21, embodiment 14 has the body regions of transistors Qr1-Qr12 connectedto its own source region. More specifically, the body regions oftransistors Qr1, Qr3, Qr5, and Qr7 are connected to word lines WL0-WL3.The body regions of transistors Qr2, Qr4, Qr6 and Qr8 are connected tothe ground node. The body regions of transistors Qr9-Qr12 are connectedto the node located at the side where the potential does not rise byself bootstrap.

FIG. 22 is a plan view showing a general structure of transistorsQr1-Qr12 of the word line drive circuit shown in FIG. 21. Referring toFIG. 22, transistors Qr1-Qr12 include an n⁺ type source region 1, an n⁺type drain region 2, a p type body region 3, a gate electrode 4, and ap⁺ type common region 38. Common region 38 is formed adjacent to sourceregion 1 and body region 3. Therefore, body region 3 is connected tosource region 1 via common region 38 to be electrically fixed.

Because embodiment 14 has the body regions of transistors Qr1, Qr3, Qr5and Qr7 connected to word lines WL0-WL3, the potentials of the bodyregions 3 follow the potentials of word lines WL0-WL3. This suppressesgeneration of a body effect in transistors Qr1, Qr3, Qr5 and Qr7, sothat the potentials of word lines QL0-WL3 promptly rise.

It is not necessary to provide a body fix line since the body regions 3of transistors Qr1-Qr12 are connected to its own source region.Accordingly, the layout area of embodiment 14 is smaller than that ofembodiment 13 of FIG. 20.

Embodiment 15

FIG. 23 is a circuit diagram showing a structure of a boost signalpredecode circuit according to a embodiment 15 of the present invention.This boost signal predecode circuit serves to provide boost signalsRX0-RX3 to the word line drive circuit shown in FIGS. 20 and 21.

Referring to FIG. 23, this boost signal predecode circuit includestransistors Qr13-Qr15, and inverters I1 and I2. Transistors Qr13 andQr14 are connected in series. Boost signal RX which is an output of theboost voltage generation circuit is provided to the drain electrode oftransistor Qr13. Row address signal X is applied to one source/drainelectrode of transistor Qr15 via inverters I1 and I2. The output ofinverter I1 is provided to the gate electrode of transistor Qr14.

This boost signal predecode circuit is activated in response to a rowaddress signal X. When a boost signal RX is applied during itsactivation, the gate potential of transistor Qr13 rises by selfbootstrap, so that transistor Qr13 attains a complete conductive state.Therefore, the applied boost signal RX is output as boost signalsRX0-RX3 via transistor Qr13.

As apparent from the above-described operation, a voltage greater thanthe power supply voltage is applied between the source and drain oftransistors Qr13-Qr15. In embodiment 15, ground potential V_(SS) isapplied to the body regions of transistors Qr13-Qr15, so that the bodyregion is electrically fixed. This causes increase in the breakdownvoltage between the source and drain of transistors Qr13-Qr15, so thatthis boost signal predecode circuit operates properly.

Embodiment 16

FIG. 24 is a circuit diagram showing a structure of a boost signalpredecode circuit of a DRAM according to embodiment 16 of the presentinvention. Referring to FIG. 24, the body regions of transistorsQr13-Qr15 are connected to its own source region.

In embodiment 16, the body region of transistor Qr13, in particular, isconnected to its own source region. Therefore, the potential of the bodyregion of transistor Qr13 rises following the output boost signalsRX0-RX3. This suppresses generation of the body effect in transistorQr13, so that boost signals RX0-RX3 rise promptly. It is not necessaryto provide a body fix line since each body region is connected to itsown source region. Therefore, the layout area of embodiment 16 issmaller than that of embodiment 15 of FIG. 23.

Embodiment 17

FIG. 25 is a plan view showing a structure of an N channel MOS capacitorof a DRAM according to embodiment 17 of the present invention. This MOScapacitor is used in a word line drive circuit, a boost signal predecodecircuit, a circuit for generating a voltage V_(PP) which is a boostedpower supply voltage, and the like.

Referring to FIG. 25, this MOS capacitor includes an n⁺ type sourceregion 1, a p type body region 3 enclosed by source region 1, a gateelectrode 4, and a p⁺ type common region 38. Common region 38 isinserted in a part of source region 1. In other words, common region 38is formed adjacent to source region 1 and body region 3. Therefore, bodyregion 3 is connected to source region 1 via common region 38. Thiscauses body region 3 to be fixed electrically, so that this MOScapacitor operates stably. Furthermore, it is not necessary to provide abody fix line since body region 3 is connected to source region 1 viacommon region 38 which is partially inserted into source region 1.Therefore, the layout area of embodiment 17 is similar to that of theconventional case.

The present invention is not limited to the above-described embodiment17 where common region 38 is inserted into a portion of source region 1.Source region 1 and common region 38 may be connected to the body fixline via a contact hole by forming one contact hole above the joiningportion of source region 1 and common region 38. This provides theadvantage that body region 3 can be electrically fixed even when thepotential of source region 1 becomes higher than the potential of commonregion 38.

Embodiment 18

FIG. 26 is a plan view showing a structure of a P channel MOS capacitorin a DRAM according to embodiment 18 of the present invention. Referringto FIG. 26, this P channel MOS capacitor includes a p⁺ type sourceregion 1, an n type body region 3 enclosed by source region 1, a gateelectrode 4, and an n⁺ type common region 38. Embodiment 18 has theconductivity type of each region opposite to those in embodiment 17 ofFIG. 25.

Embodiment 19

FIG. 27 is a plan view showing a structure of an N channel MOS capacitorof a DRAM according to embodiment 19 of the present invention. Referringto FIG. 27, this N channel MOS capacitor includes two n⁺ type sourceregion 1, a p type body region 3 located between these source regions 1,a gate electrode 4, and a p⁺ type contact region 31. The two sourceregions 1 are connected to each other. Contact region 31 is insertedinto a portion of body regions, and is formed adjacent to only that bodyregion 3.

A potential identical to that applied to source region 1 is provided tocontact region 31, whereby body region 3 is connected to source region 1via contact region 31. Therefore, this N channel MOS capacitor operatesstably since that body region 3 is electrically fixed.

Because a potential identical to that applied to source region 1 isprovided to contact region 31 in embodiment 19, ground potential V_(SS)or negative potential V_(BB) may be applied to contact region 31.

Embodiment 20

FIG. 28 is a plan view showing a structure of a P channel MOS capacitorof a DRAM according to embodiment 20 of the present invention. Referringto FIG. 28, this P channel MOS capacitor includes two p⁺ type sourceregions 1, an n type body region 3 located therebetween, a gateelectrode 4, and an n⁺ type contact region 31. A predetermined potentialis provided to contact region 31, whereby body region 3 is electricallyfixed. Embodiment 20 has the conductivity type of each region oppositeto that of embodiment 19 of FIG. 27.

Embodiment 21

FIG. 29 is a circuit diagram showing a structure of a boost power supplygeneration circuit of a DRAM according to embodiment 20 of the presentinvention. Referring to FIG. 29, this boost power supply generationcircuit includes three MOS capacitors Cbs1-Cbs3, and an N channel MOStransistor Qbs at the final stage. This boost power supply generationcircuit generates a boost potential V_(PP) higher than power supplypotential V_(CC) in response to a clock signal CK.

In transistor Qbs of this boost power supply generation circuit, thepotential of the drain electrode (output node) is always higher than thepotential of the source electrode. Therefore, the body region oftransistor Qbs is connected to its own source region. As a result, thebreakdown voltage between the source and drain of transistor Qbs isincreased. There is also the advantage that a body fix line does nothave to be provided since the body region is connected to the sourceregion. Therefore, the layout area according to embodiment 21 issubstantially equal to that of a conventional one.

Embodiment 22

FIG. 30 is a circuit diagram showing a structure of an outputpreamplifier and a write circuit of a DRAM according to embodiment 22 ofthe present invention. Referring to FIG. 30, a current mirror typeoutput preamplifier 40 includes P channel MOS transistors Qp5-Qp11, andN channel MOS transistors Qn5-Qn12. An output preamplifier is generallysusceptible to a kink since the potentials of input/output lines IO and/IO are amplified in an analog manner. Therefore, the body regions of Pchannel MOS transistors Qp5-Qp11 are connected to its own source region.The body regions of N channel MOS transistor Qn5-Qn12 are connected toits own source region.

Because embodiment 22 has the body regions of transistors Qp5-Qp11 andQn5-Qn12 electrically fixed, a kink will not be generated in thesetransistors. Accordingly, output preamplifier 40 can amplify stably thepotential of input/output lines IO and /IO.

Write circuit 41 includes four N channel MOS transistors Qn13-Qn16.Ground potential V_(SS) is applied to the body region of transistorsQn13-Q16, so that these body regions are electrically fixed. Therefore,a great leakage current will not flow between the source and drain oftransistors Qn13-Qn16.

Embodiment 23

FIG. 31 is a circuit diagram showing a structure of an input/output lineprecharge circuit and an input/output line equalize circuit of a DRAMaccording to embodiment 23. Referring to FIG. 31, an input/output lineprecharge circuit 42 includes P channel MOS transistors Qp21 and Qp22,and N channel MOS transistors Qn21 and Qn22. P channel MOS transistorQp21 and N channel MOS transistor Qn21 form a transfer gate. Also, Pchannel MOS transistor Qp22 and N channel MOS transistor Qn22 also forma transfer gate. Input/output line precharge circuit 42 responds to aprecharge signal YN to precharge input/output lines IO and /IO to apredetermined potential.

The input/output line equalize circuit includes a P channel MOStransistor Qp20 and an N channel MOS transistor Qn20. Transistors Qp20and Qn20 form a transfer gate. This input/output line equalize circuitresponds to input/output line equalize signals IOEQ and /IOEQ to equalthe potential of input/output lines IO and /IO to each other.

In embodiment 23, power supply potential V_(CC) is applied to the bodyregions of P channel MOS transistors Qp20-Qp22. Ground potential V_(SS)is applied to the body regions of N channel MOS transistors Qn20-Qn22.As a result, the body regions of transistors Qp20-Qp22 and Qn20-Qn22 areelectrically fixed. Therefore, a great amount of leakage current willnot flow between the source and drain of these transistors. Thus,correct data is transmitted via input/output lines IO and /IO.

Embodiment 24

FIG. 32 is a circuit diagram showing a row address buffer in a DRAMaccording to embodiment 24 of the present invention. Referring to FIG.32, this dynamic latch type row address buffer includes P channel MOStransistors Qp25-Qp28, and N channel MOS transistors Qn25-Qn30. Thisaddress buffer responds to an external address signal ext. An togenerate internal row address signals RAn and /RAn. This address buffercompares external address signal ext.An to a reference signal VREF tomake determination whether external address signal ext.An attains a Hlevel or a L level.

In embodiment 24, the body regions of transistors Qp25-Qp28 areconnected to the source electrode. Ground potential V_(SS) is applied tothe body regions of transistors Qn25, Qn26, Qn29 and Qn30. This dynamiclatch type row address buffer responds to a fall of control signal/RADBE to latch an address signal. In an active state, a reverse voltagewill not be applied to the PN junction between the body region and thesource region. Therefore, it is possible to connect the body regions oftransistors Qn27 and Qn28 to its own source region.

Because the body regions of transistors Qp25-Qp28 and Qn25-Qn30 areelectrically fixed, this row address buffer carries out an analogoperation stably. Because transistors Qp27 and Qp28 are impervious tothe body effect, this row address buffer can make determination whetherexternal address signal ext.An attains a H level or a L level stably andspeedily.

Embodiment 25

FIG. 33 is a circuit diagram showing a structure of a column addressbuffer of a DRAM according to embodiment 25 of the present invention.Referring to FIG. 33, this column address buffer includes P channel MOStransistors Qp31-Qp34, and N channel MOS transistors Qn31-Qn34.Transistors Qp31, Qp32, Qn31 and Qn32 form an NOR circuit. TransistorsQp33, Qp34, Qn33 and Qn34 form a clocked inverter of a subsequent stage.This column address buffer responds to an external address signal ext.Anto generate internal column address signals CAn and /CAn.

The body regions of all transistors Qp31, Qp32, Qn31, Qn32 forming thisNOR circuit are connected to its own source region. Therefore, groundpotential V_(SS) is applied to the body region of N channel MOStransistors Qn31 and Qn32. The body regions of all transistors Qp33,Qp34, Qn33 and Qn34 forming the inverter are set to a floating state.

In embodiment 25, the body regions of transistors Qp31, Qp32, Qn31, andQn32 of the input stage are electrically fixed, so that correctdetermination can be made whether external address signal ext.An attainsa H level or a L level. Furthermore, because the body regions oftransistors Qp33, Qp34, Qn33 and Qn34 forming the clocked inverter ofthe subsequent stage, and the body regions of transistors forming otherlogical gates are set to a floating stage, it is not necessary toprovide a body fix line. Therefore, increase in the layout area issuppressed to a minimum.

Embodiment 26

FIG. 34 is a circuit diagram showing a structure of a column addressbuffer of a DRAM according to embodiment 26 of the present invention.Referring to FIG. 34, embodiment 26 has a negative potential V_(BB)applied to the body regions of N channel MOS transistors Qn31 and Qn32forming an NOR circuit. As described above, a negative potential V_(BB)instead of ground potential V_(SS) may be applied to the body regions ofN channel MOS transistors Qn31 and Qn32.

Embodiment 27

FIG. 35 is a circuit diagram showing a column address buffer of a DRAMaccording to embodiment 27 of the present invention. Embodiment 27 hasthe body regions of transistors Qp33, Qp34, Qn33 and Qn34 forming theinverter connected to its own source region. According to embodiment 27,the body regions of the transistors forming the input stage and theclocked inverter of the subsequent stage are electrically fixed.Therefore, although there is a slight increase in the layout area, thiscolumn address buffer operates stably in comparison with the case wherethe body regions thereof are not fixed.

Embodiment 28

FIG. 26 is a circuit diagram showing a structure of a column addressbuffer of a DRAM according to embodiment 28 of the present invention.Differing from the column address buffer shown in FIG. 27, embodiment 28has negative potential V_(BB) applied to the body regions of N channelMOS transistors Qn31 and Qn32. Thus, a negative potential V_(BB) may beapplied instead of ground potential V_(SS) to the body regions of Nchannel MOS transistors Qn31 and Qn32.

Embodiment 29

FIG. 37 is a circuit diagram showing a structure of a clock input bufferof a DRAM according to embodiment 29 of the present invention. Referringto FIG. 37, this clock input buffer includes P channel MOS transistorsQp35-Qp37, an N channel MOS transistor Qn35, and inverters I3-I5. Thisclock input buffer responds to external row address strobe signalext./RAS of a MOS level or a TTL level to generate internal row addressstrobe signals RAS and /RAS.

In embodiment 29, the body regions of transistors Qp35-Qp37 of the inputstage and transistor Qn35 are connected to its own source region.Therefore, ground potential V_(SS) is applied to the body region of Nchannel MOS transistors Qn35.

Because the body regions of transistors Qp35-Qp37 and Qn35 areelectrically fixed, accurate determination can be made whether externalrow address strobe signal ext./RAS attains a H level or a L level.Furthermore, it is not necessary to provide a body fix line since thebody regions of transistors Qp35-Qp37 and Qn35 are connected to its ownsource region. Accordingly, the layout area of this clock input bufferis similar to that of a conventional case.

Embodiment 30

FIG. 38 is a circuit diagram showing a structure of a clock input bufferof a DRAM according to embodiment 30 of the present invention. Inembodiment 30, negative potential V_(BB) is applied to the body regionof N channel MOS transistor Qn35 of the first input stage. Therefore,negative potential V_(BB) may be applied, instead of ground potentialV_(SS), to the body region of N channel MOS transistor Qn35.

Embodiment 31

FIG. 39 is a circuit diagram showing a structure of a clock input bufferof a DRAM according to embodiment 31 of the present invention. Similarto embodiment 29 of FIG. 37, this clock input buffer of FIG. 39 includesP channel MOS transistors Qp35-Qp37, an N channel MOS transistor Qn35,and inverters I3-I5. Inverter I4 includes a P channel MOS transistorQp38, and an N channel MOS transistor Qn38. Inverter I5 includes a Pchannel MOS transistor Qp39, and an N channel MOS transistor Qn39.

In embodiment 31, the body regions of P channel MOS transistors Qp38 andQp39 forming inverters I4 and I5 are connected to its own source region.Also, a negative potential V_(BB) is applied to the body regions of Nchannel MOS transistors Qn38 and Qn39 forming inverters I4 and I5.

Because embodiment 31 has the body regions of transistors Qp38, Qp39,Qn38 and Qn39 of the last stage electrically fixed, the clock skew ofinternal row address strobe signals RAS and /RAS generated by this clockinput buffer is reduced.

Embodiment 39

FIG. 40 is a circuit diagram showing a structure of a clock input bufferof a DRAM according to embodiment 32 of the present invention. Inembodiment 32, the body regions of N channel MOS transistors Qn38 andQn39 are connected to its own source region.

Because the body regions of N channel MOS transistors Qn38 and Qn39 areconnected to its own source region in embodiment 32, it is not necessaryto provide a body fix line. Therefore, increase of the layout area canbe suppressed to a minimum. Ground potential V_(SS) may be applied,instead of negative potential V_(BB), to the body regions of transistorsQn38 and Qn39 of the last stage.

Embodiment 33

FIG. 41 is a circuit diagram showing a structure of a sense amplifierdrive circuit of a DRAM according to embodiment 33 of the presentinvention. Referring to FIG. 41, this sense amplifier drive circuitincludes a plurality of inverters connected in series, and a pluralityof MOS capacitors Csp1-Csp3, and Csn1-Csn3. This sense amplifier drivecircuit responds to a boost signal RX to generate control signals SOF,SON, and /SOP to drive sense amplifier 20.

FIG. 42 is a timing chart showing the operation of the sense amplifierdrive circuit of FIG. 41. Referring to FIG. 42, control signal SOF risesafter a predetermined time period from the rise of boost signal RX.Control signal SON rises after a predetermined time period from the riseof control signal SOF. Then, control signal /SOP falls after apredetermined time period from the rise of control signal SON.

In the sense amplifier drive circuit of embodiment 33, the body regionsof all MOS capacitors Csp1-Csp3 and Csn1-Csn3 are electrically fixed.Therefore, the threshold values of MOS capacitors Csp1-Csp3 andCsn1-Csn3 do not become unstable, so that the time period from a rise ofboost signal RX to a rise of control signal SOF, or the time period froma rise of control signal SOF to a rise of control signal SON will not bereduced. As a result, there is no reduction in the operation margin ofsense amplifier 20.

In embodiment 33, power supply potential V_(CC) is applied to the bodyregions of P channel MOS capacitors Csp1-Csp3. Ground potential V_(SS)is applied to the body regions of N channel MOS capacitors Csn1-Csn3.However, negative potential V_(BB) may be applied, instead of groundpotential V_(SS), to the body regions of N channel MOS capacitorsCsn1-Csn3.

Embodiment 34

FIG. 43 is a circuit diagram showing a structure of a CAT (ColumnAddress Transition) circuit of a DRAM according to embodiment 34 of thepresent invention. Referring to FIG. 43, this CAT circuit includes threeinverters I20-I22, three NOR circuits NR1-NR3, two P channel MOScapacitors Ctp1 and Ctp2, and N channel MOS capacitors Ctn1 and Ctn2.This CAT circuit responds to control signal CAD to generate a controlsignal CAT. Here, power supply potential V_(CC) is applied to the bodyregions of P channel MOS capacitors Ctp1 and Ctp2. Ground potentialV_(SS) is applied to the body regions of N channel MOS capacitors Ctn1and Ctn2.

FIG. 44 is a timing chart showing an operation of the CAT circuit ofFIG. 43. Referring to the timing chart of FIG. 43, the potential of anoutput node A of NOR circuit NR1 falls immediately when control signalCAD rises. The potential of an output node B of NOR circuit NR3 risesafter a predetermined time period from the fall of the potential of nodeA. Then, the potential of node B falls immediately when control signalCAD falls. The potential of node A rises after a predetermined timeperiod from the fall of the potential of node B.

Control signal CAT rises immediately when the potential of node A falls.Control signal CAT falls immediately when the potential of node B rises.Also, control signal CAT rises immediately when the potential of node Bfalls. Control signal CAT falls immediately when the potential of node Arises.

If the body regions of MOS capacitors Ctp1, Ctp2, Ctn1 and Ctn2 attain afloating state here, the threshold values of the capacitors thereofbecomes unstable due to change in the potential of the body region.There is a possibility that the capacitance of the capacitors thereofbecomes unstable. This causes delay in the fall time of control signalCAT1 or an early fall of control signal CAT2 as shown in the timingchart of FIG. 44. The operation margin of this CAT circuit is reducedwhen control signal CAT2 is delayed in its fall.

In embodiment 34, the body regions of MOS capacitors Ctp1, Ctp2, Ctn1and Ctn2 are electrically fixed. Therefore, a control signal CAT that isalways stable is generated.

Embodiment 35

FIG. 45 is a circuit diagram showing a structure of an N—N buffer of aDRAM according to embodiment 35 of the present invention. Such an N—Nbuffer is used as a data output buffer of a DRAM.

Referring to FIG. 45, this N—N buffer includes N channel MOS transistorsQnn1 and Qnn2 connected in series. The body regions of N channel MOStransistors Qnn1 and Qnn2 are connected to its own source region.Therefore, the body region of N channel MOS transistor Qnn1 is connectedto output node OUT.

In this N—N buffer, complementary signals Do and /Do are connected tothe gate electrodes of transistors Qnn1 and Qnn2, respectively. Whensignal Do attains a H level and signal /Do attains a L level, transistorQnn1 attains a conductive state, and transistor Qnn2 attains anonconductive state. Therefore, a signal of a H level is output.

In the N—N buffer of embodiment 35, the body regions of N channel MOStransistors Qnn1 and Qnn2 are electrically fixed, so that the thresholdvalue will not become unstable. As a result, a great leakage currentwill not flow between the source and drain of transistors Qnn1 and Qnn2.Leakage current will not flow outwards via transistor Qnn1, or flow infrom an external source via transistor Qnn2.

Because the body regions of transistors Qnn1 and Qnn2 are connected toits own source region in this N—N buffer, there is no increase in thelayout area. Furthermore, because the body region of transistor Qnn1 isconnected to the output node, the potential of the body region thereoffollows the potential of the output node. Therefore, the threshold valuewill not rise due to a body effect in transistor Qnn1. The output signalof the present N—N buffer rises promptly to the level of power supplypotential V_(CC).

Embodiment 36

FIG. 46 is a circuit diagram showing a structure of an N—N buffer of aDRAM according to embodiment 36 of the present invention. In embodiment36, ground potential V_(SS) is supplied to the body regions oftransistors Qnn1 and Qnn2. Thus, ground potential V_(SS) may be applied,instead of the source potential, to the body regions of transistors Qnn1and Qnn2.

Embodiment 37

FIG. 47 is a circuit diagram showing a structure of a 2-input NANDcircuit of a DRAM according to embodiment 37 of the present invention.Such a NAND circuit is used in various places of a DRAM such as in aclock input buffer that generates an internal row address strobe signal/RAS.

Referring to FIG. 47, this CMOS type NAND circuit includes two inputterminals. This NAND circuit includes P channel MOS transistorsQgp1-Qgp2 connected in parallel between the power supply node and outputnode 50, and N channel MOS transistors Qgn1 and Qgn2 connected in seriesbetween output node 50 and ground node 51. An input signal IN1 isapplied to the gate electrodes of transistors Qgp1 and Qgn1. An inputsignal IN2 is applied to the gate electrodes of transistors Qgp2 andQgn2. Output signal OUT is provided from output node 50.

In the present NAND circuit, the body regions of P channel MOStransistors Qgp1 and Qgp2 are set to a floating state, whereas the bodyregions of N channel MOS transistors Qgn1 and Qgn2 are connected to itsown source region. Therefore, these body regions are electrically fixed.As a result, the threshold value of transistor Qgn1 is stable andreduced, so that this NAND circuit operates at high speed. Thus, thisNAND circuit operates properly even when power supply potential V_(CC)is low.

FIG. 48 is a plan view showing a structure of N channel MOS transistorsQgn1 and Qgn2 in the NAND circuit of FIG. 47. Referring to FIG. 48,transistor Qgn1 includes an n⁺ type drain region 52, an n⁺ typesource/drain region 53, a p type body region 57, and a gate electrode59. Transistor Qgn2 includes an n⁺ type source/drain region 53 common totransistor Qgn1, an n⁺ source region 54, a p type body region 58, and agate electrode 60. Drain region 52 of transistor Qgn1 is connected tooutput node 50 of aluminum via contact hole CH. Source region 54 oftransistor Qgn2 is connected to ground node 51 of aluminum via contacthole CH.

A p⁺ type common region 55 is formed in a portion of source/drain region53. Contact hole CH is formed above the junction portion of source/drainregion 53 and common region 55 with an intermediate layer 61 of aluminumtherebetween. Therefore, body region 57 is connected to source/drainregion 53 via common region 55 to be electrically fixed.

A p⁺ type common region 56 is formed in a portion of source region 54.Contact hole CH is formed above the junction portion of source region 54and common region 56. Therefore, body region 58 is connected to sourceregion 54 via common region 56 to be electrically fixed.

Embodiment 38

FIG. 49 is a plan view showing another structure of N channel MOStransistors Qgn1 and Qgn2 in the NAND circuit of FIG. 47. Referring toFIG. 49, an intermediate layer 62 of polysilicon is formed on drainregion 52, source/drain region 53, and source region 54 in embodiment38. Intermediate layer 62 serves as an etching stopper. The SOIsubstrate is prevented from being etched during the formation of contacthole CH by an etching process.

Embodiment 39

FIG. 50 is a plan view showing another structure of N channel MOStransistors Qgn1 and Qgn2 of the NAND circuit of FIG. 47. Referring toFIG. 50, a source/drain region 65 partially protrudes between gateelectrodes 59 and 60. A p⁺ type common region 66 is formed adjacent tothe protruding portion of source/drain region 65. Contact hole CH isformed above the junction portion of the protruding portion ofsource/drain region 65 and common region 66 with an intermediate layer67 of aluminum therebetween.

In embodiment 39, body region 57 is connected to source region 65 viacommon region 66 to be electrically fixed. Because a contact hole is notformed between gate electrodes 59 and 60, the distance between gateelectrodes 59 and 60 can be reduced.

Embodiment 40

FIG. 51 is a plan view showing another structure of N channel MOStransistors Qgn1 and Qgn2 of the NAND circuit shown in FIG. 47.Referring to FIG. 51, an intermediate layer 68 of polysilicon is formedabove drain region 52, source/drain region 65, and source region 54 inembodiment 40. Therefore, the SOI substrate will not be etched duringthe formation of contact hole CH which is carried out by an etchingprocess.

Embodiment 41

FIG. 52 is a circuit diagram showing a structure of a 3-input NANDcircuit of a DRAM according to embodiment 41 of the present invention.Referring to FIG. 52, this 3-input NAND circuit includes P channel MOStransistors Qgp5, Qgp4, Qgp3 connected in parallel between the powersupply node and an output node 70, and N channel MOS transistors Qgn3,Qgn4, and Qgn5 connected in series between output node 70 and a groundnode 71. An input signal IN1 is provided to the gate electrodes oftransistors Qgp3 and Qgn3. An input signal IN2 is provided to the gateelectrodes of transistors Qgp4 and Qgn4. An input signal IN3 is providedto the gate electrodes of transistors Qgp5 and Qgn5. An output signalOUT is provided from output node 70.

In this NAND circuit, the body regions of transistors Qgp3-Qgp5 are setto a floating state, and the body regions of transistors Qgn3-Qgn5 areconnected to the source region to be electrically fixed. This causesreduction in the threshold value of transistors Qgn3 and Qgn4. Thus,this 3-input NAND circuit operates at high speed. Furthermore, it is notnecessary to provide a body fix line since the body regions oftransistors Qgp3-Qgp5 are set to a floating state. Thus, there is almostno increase in the layout area.

FIG. 53 is a plan view showing a structure N channel MOS transistorsQgn3-Qgn5 in the 3-input NAND circuit shown in FIG. 52. Referring toFIG. 53, transistor Qgn3 includes an n⁺ type drain region 72, an n⁺ typesource/drain region 73, a p type body region 79, and a gate electrode82. Transistor Qgn4 includes a source/drain region 37 common totransistor Qgn3, an N⁺ source/drain region 74, and a p type body region80. Transistor Qgn5 includes a source/drain region 74 common totransistor Qgn4, an n⁺ type source region 75, a p type body region 81,and a gate electrode 84.

Drain region 72 of transistor Qgn3 is connected to output node 70 viatwo contact holes CH. p⁺ type common region 76 is formed in a part ofsource/drain region 73. Therefore, body region 79 of transistor Qgn3 isconnected to source/drain region 73 via common region 76 to beelectrically fixed. Contact hole CH is formed on the junction portion ofsource/drain region 73 and common region 76 with an intermediate layer85 of aluminum therebetween.

p⁺ type common region 77 is formed in a portion of source/drain region74. Therefore, body region 80 of transistor Qgn4 is connected to sourceregion 74 via common region 77 to be electrically fixed. Contact hole CHis formed on the junction portion of source/drain region 74 and commonregion 77 with an intermediate layer 85 of aluminum therebetween. Sourceregion 75 of transistor Qgn5 is connected to ground node 71 via twocontact holes CH. p⁺ type common region 78 is formed at a portion ofsource region 75. The body region of transistor Qgn5 is connected tosource region 75 via common region 78 to be electrically fixed.

Embodiment 42

FIG. 54 is a plan view showing another structure of N channel MOStransistors Qgn3-Qgn5 in the 3-input NAND circuit of FIG. 52. Referringto FIG. 54, embodiment 42 has intermediate layer 86 of polysiliconformed on drain region 72, source/drain regions 73, 74 and source region75. Therefore, the SOI substrate will not be etched during the formationof contact hole CH carried out by an etching process.

Embodiment 43

FIG. 55 is a plan view showing another structure of N channel MOStransistors Qgn3-Qgn5 in the 3-input NAND circuit of FIG. 52. Referringto FIG. 55, a source/drain region 90 protrudes from between gateelectrodes 82 and 83 in embodiment 43. A source/drain region 91 alsoprotrudes from between gate electrodes 83 and 84. A p⁺ type commonregion 92 is formed adjacent to the protruding portion of source/drainregion 90. Therefore, body region 79 of transistor Qgn3 is connected tosource/drain region 90 via common region 92 to be electrically fixed. Ap⁺ type common region 93 is formed adjacent to the protruding portion ofsource/drain region 91. Therefore, body region 80 of transistor Qgn4 isconnected to source/drain region 91 via common region 93 to beelectrically fixed. A p⁺ type common region 78 is formed at a portion ofsource region 75. Therefore, body region 81 of transistor Qgn5 isconnected to source region 75 to be electrically fixed.

A contact hole CH is formed on the junction portion of source/drainregion 90 and common region 92 with an intermediate layer 94 of aluminumtherebetween. A contact hole CH is formed on the junction portion ofsource/drain region 91 and common region 93 with an intermediate layer94 of aluminum therebetween.

Because a contact hole is not formed between gate electrodes 82 and 83in embodiment 43, the distance between gate electrodes 82 and 83 can bemade shorter. Also, the distance between gate electrodes 83 and 84 canbe made shorter since a contact hole is not formed therebetween.

Embodiment 44

FIG. 56 is a plan view showing another structure of N channel MOStransistors Qgn3-Qgn5 in the 3-input NAND circuit of FIG. 52. Referringto FIG. 56, an intermediate layer 95 of polysilicon is formed on drainregion 72, source/drain regions 90, 91 and source region 75 inembodiment 44. Therefore, the SOI substrate is not etched during theformation of contact hole CH which is carried out by an etching process.

Embodiment 45

FIG. 57 is a circuit diagram showing a structure of a 3-input NANDcircuit of DRAM according to embodiment 45 of the present invention.Referring to FIG. 57, the body region of transistor Qgn3 is connected tothe source/drain region common to transistors Qgn4 and Qgn5. Also, thebody regions of transistors Qgn4 and Qgn5 are set to a floating state.When the potential of output node 70 falls to a L level in such a NANDcircuit, the potential of the body region of transistor Qgn3 is alwaysat the level of the ground potential.

It is appreciated from embodiment 45 that at least the body region oftransistor Qgn3 directly connected to output node 70 should beelectrically fixed. The body region of transistor Qgn3 may be connected,not to its own source region, but to the source/drain region common totransistors Qgn4 and Qgn5.

In embodiment 45, the potential applied to the body region of transistorQgn3 is not constant, and rises according to the rise of the potentialof output node 70. Therefore, a body effect is not generated in thistransistor Qgn3, so that this 3-input NAND circuit operates at highspeed.

Embodiment 46

FIG. 58 is a circuit diagram showing a structure of a 2-input NANDcircuit of a negative logic (a 2-input NOR circuit of a positive logic)of a DRAM according to embodiment 46 of the present invention. Referringto FIG. 58, this 2-input NAND circuit includes N channel MOS transistorsQgn6 and Qgn7 connected in parallel between ground node 51 and outputnode 50, and P channel MOS transistor Qgp6 and Qgp8 connected in seriesbetween output node 50 and the power supply node. An input signal IN1 isapplied to the gate electrodes of transistors Qgn7 and Qgp6. An inputsignal IN2 is applied to the gate electrodes of transistors Qgn6 andQgp7. An output signal OUT is provided from output node 50.

In this NAND circuit, the body regions of transistors Qgn6 and Qgn7 areset to a floating state. The body regions of transistors Qgp6 and Qgp7are connected to its own drain region. Therefore, the body region oftransistor Qgp6 is supplied with a drain potential that rises inresponse to the rise of output signal OUT. A constant ground potentialV_(SS) is applied to the drain region of transistor Qgp7. Therefore,this NAND circuit operates at high speed since the threshold value oftransistor Qgp6 is reduced. This NAND circuit can operate properly evenwhen the power supply potential V_(CC) is low.

Embodiment 47

FIG. 59 is a circuit diagram showing a structure of a 2-input NANDcircuit of a DRAM according to embodiment 47 of the present invention.Referring to FIG. 59, power supply potential V_(CC) is applied to thebody region of P channel MOS transistor Qgp6 to be electrically fixed inthis NAND circuit. Although a body effect is generated in transistorQgp6 in embodiment 47, the body region of transistor Qgp6 may besupplied with power supply potential V_(CC) instead of its own drainpotential.

Embodiment 48

FIG. 60 is a sectional view of a planar type memory cell portion takenalong the bit line direction of a DRAM according to embodiment 48 of thepresent invention. FIG. 61 is a sectional view of the memory cellportion of FIG. 60 taken along the word line direction.

As shown in FIGS. 60 and 61, a source/drain region 44, a LOCOS oxidefilm 5, a gate electrode 4, and a cell plate electrode 45 are formed onan SOI substrate 6. Gate electrode 4 and cell plate electrode 45 areformed within first interlayer insulation film 33. Here, twosource/drain regions 44, body region 3 therebetween, and gate electrode4 form one N channel MOS transistor. One source/drain region 44, bodyregion 3, and cell plate electrode 45 form one N channel MOS capacitor.

Source/drain region 44 common to the two transistors is connected to bitline BL via an intermediate layer 32 of a polypad. A second interlayerinsulation film 34 is formed on first interlayer insulation film 33 andintermediate layer 32. Bit line BL is formed on second interlayerinsulation film 34. Bit line BL is connected to intermediate layer 32via a contact hole. A third interlayer insulation film 35 is formed onbit line BL. On third interlayer insulation film 35, a pillar word line46 of aluminum is formed. Pillar word line 46 is connected to word lineWL that forms gate electrode 4 via a contact hole at every constantinterval. Therefore, when drive voltage is supplied to word line WL, asignal propagation delay generated by word line WL is reduced.

Referring to FIG. 61, contact region 31 is formed in a portion of bodyregion 3 of the transistor. Therefore, body region 3 is connected tobody fix line 30 via contact region 31 and intermediate layer 32 to beelectrically fixed. Because body region 3 of the transistor forming amemory cell is electrically fixed, the threshold value of thattransistor does not become unstable, and a great leakage current willnot flow between the source and drain. Therefore, the data retainingtime period in this memory cell is increased. Even when charge isgenerated in silicon substrate 7 due to introduction of α particles intothis SOI substrate 6, the charge will not enter body region 3 becausebody region 3 and silicon substrate 7 are electrically isolated byburied oxide layer 8. Because this body region 3 is extremely thin,there is almost no generation of charge caused by α particles in bodyregion 3. Therefore, there is almost no generation of the so-called softerror.

Embodiment 49

FIG. 62 is a sectional view of a memory cell portion of a DRAM takenalong the bit line direction according to embodiment 49 of the presentinvention. FIG. 63 is a sectional view of the memory cell portion ofFIG. 62 taken along the word line direction. In embodiment 49 shown inFIGS. 62 and 63, a field shield electrode 47 is formed instead of aLOCOS oxide film on SOI substrate 6. This field shield electrode 47 isformed in first interlayer insulation film 33.

Ground potential V_(SS) or a negative potential is applied to fieldshield electrode 47, whereby the portion of SOI active layer 9 beneathfield shield electrode 47 is rendered non-conductive. Therefore, thistransistor and this capacitor are electrically isolated from an adjacentelement. It is appreciated from embodiment 49 that elements such as atransistor may be isolated, not by a LOCOS, but by other isolationmethods such as a field shield.

Embodiment 50

FIG. 64 is a sectional view of a memory cell portion of a DRAM takenalong the bit line direction according to embodiment 50 of the presentinvention. FIG. 64 shows a stacked type memory cell isolated by LOCOSoxide film 5.

Referring to FIG. 64, a source/drain region 44, a LOCOS oxide film 5,and a gate electrode 4 are formed on SOI substrate 6. The twosource/drain regions 44, body region 3 located therebetween, and gateelectrode 4 form one N channel MOS transistor.

Source/drain region 44 common to the two transistors is connected to bitline BL via intermediate layer 32. A storage node 48 and a cell plateelectrode 45 are formed on the other source/drain region 44 of thattransistor. Storage node 48 and cell plate 45 form the electrode of thecapacitor. Therefore, the above-referred N channel MOS transistor andcapacitor form a memory cell.

A contact region (not shown) is formed in a portion of body region 3 ofa transistor. Therefore, body region 3 is connected to a body fix line(not shown) via that contact region to be electrically fixed.

Embodiment 51

FIG. 65 is a sectional view of a memory cell unit taken along the bitline direction of a DRAM according to embodiment 51 of the presentinvention. FIG. 65 shows a stacked type memory cell isolated by a fieldshield.

Referring to FIG. 65, embodiment 51 has a field shield electrode 47formed instead of a LOCOS oxide film on SOI substrate 6. Also, a contactregion (not shown) is formed at a portion of body region 3 of thistransistor. Therefore, body region 3 is connected to a body fix line(not shown) via the contact region. Ground potential V_(SS) or V_(BB) isapplied to the body fix line. Therefore, body region 3 of the transistoris electrically fixed.

Embodiment 52

FIG. 66 is a layout diagram showing an entire structure of a DRAMaccording to embodiment 52 of the present invention. Referring to FIG.66, this DRAM includes four memory cell arrays 11, two row decoders 12,two column decoders 13, and a peripheral circuit 99. Each row decoder 12is disposed between two memory cell arrays 11. Each column decoder 13 isdisposed at one side of two memory cell arrays 11.

In embodiment 52, the elements in memory cell array 11 are isolated by aLOCOS. A negative potential V_(BB) is applied to the body region of theN channel MOS transistor forming each memory cell in the memory cellarray to be electrically fixed.

Row decoder 12 includes a plurality of P channel MOS transistors and aplurality of N channel MOS transistors. The body region of the P channelMOS transistor in row decoder 12 is supplied with power supply potentialV_(CC), whereby the body region is electrically fixed. The body regionof the N channel MOS transistor in row decoder 12 is supplied with theground potential V_(SS), whereby the body region is electrically fixed.

Column decoder 13 includes a plurality of N channel MOS transistors. Thebody region of the N channel MOS transistor in column decoder 13 issupplied with ground potential V_(SS) to be electrically fixed.

Peripheral circuit 99 located between column decoders 13 includes aplurality of P channel MOS transistors. Power supply potential V_(CC) isapplied to the body region of the P channel MOS transistor in peripheralcircuit 99, whereby the body region is electrically fixed. The otherperipheral circuit 99 includes a plurality of N channel MOS transistors.Ground potential V_(SS) is applied to the body region of the N channelMOS transistor in peripheral circuit 99, whereby the body region iselectrically fixed.

As described above, all the body regions in the MOS transistors in thisDRAM are electrically fixed. It is to be noted that the body region ofthe P channel MOS transistor is supplied with power supply potentialV_(CC). Furthermore, negative potential V_(BB) is applied to the bodyregions of the transistors in memory cell array 11 out of the N channelMOS transistors. Ground potential V_(SS) is applied to the body regionsof the other N channel MOS transistors.

Therefore, the threshold voltage of the N channel MOS transistor inmemory cell array 11 becomes greater than that of other N channel MOStransistors. Therefore, the leakage current flowing in the transistorsforming the memory cell is reduced, resulting in a longer data retainingtime period of the memory cell.

Embodiment 53

FIG. 67 is a layout diagram showing an entire structure of a DRAMaccording to embodiment 53 of the present invention. Referring to FIG.67, all the body regions in the N channel MOS transistors in memory cellarray 11 are set to a floating state.

In general, memory cell array 11 has transistors arranged at a densityhigher than that of peripheral circuit 99. Therefore, there is almost noincrease in the layout area even when a body fix line is disposed in rowdecoder 12, column decoder 13, and peripheral circuit 99. Furthermore,because it is not necessary to provide a body fix line in memory cellarray 11, the layout area is similar to that of a conventional one.

Embodiment 54

FIG. 68 is a layout diagram showing an entire structure of a DRAMaccording to embodiment 54 of the present invention. Referring to FIG.68, the elements in memory cell array 11 are isolated by a field shieldin embodiment 54. The body region in the transistor in memory cell array11 is supplied with negative potential Vbb, as in FIG. 66.

By isolating at least the elements in memory cell array 11, the bodyregion of the transistor in that memory cell array 11 can beelectrically fixed without having to provide a body fix line in memorycell array 11. Therefore, the layout area of the DRAM of embodiment 54is smaller than that of embodiment 52. Although the layout area ofembodiment 54 is substantially equal to that of embodiment 53, there isalmost no leakage current in the transistors of memory cell array 11since the body region is electrically fixed. Therefore, the dataretaining time period according to embodiment 54 is longer than that ofembodiment 53.

Embodiment 55

FIG. 69 is a layout diagram showing an entire structure of a DRAMaccording to embodiment 55. Referring to FIG. 69, the body region of theN channel MOS transistor in memory cell array is supplied with groundpotential V_(SS) to be electrically fixed. Therefore, ground potentialV_(SS) is applied to all the body regions of N channel MOS transistors,and power supply potential V_(CC) is applied to all the body regions ofP channel MOS transistors. Thus, ground potential V_(SS) may be appliedto the body region in the transistor of memory cell array 11.

Embodiment 56

FIG. 70 is a diagram showing the concept of a DRAM according toembodiment 56 of the present invention. Referring to FIG. 70, this DRAMincludes a plurality of N channel MOS transistors and a plurality of Pchannel MOS transistors. Ground potential V_(SS) is applied to the bodyregion of several transistors of the N channel MOS transistors, andnegative potential V_(BB) is applied to the body region of the other Nchannel MOS transistors. Power supply potential V_(CC) is applied to allthe body regions of all P channel MOS transistors.

Therefore, in embodiment 56, the body region of all MOS transistors areelectrically fixed. Because the threshold voltage of the transistorhaving a body region to which negative potential V_(BB) is supplied isgreater than the threshold voltage of the transistor having a bodyregion to which ground potential V_(SS) is supplied, these plurality ofN channel MOS transistors have two types of threshold voltages.

Embodiment 57

FIG. 71 is a diagram showing the concept of a DRAM according toembodiment 57 of the present invention.

Referring to FIG. 71, the body region of some N channel MOS transistorsare set to a floating state. Because it is not necessary to provide abody fix line in the N channel MOS transistor region having a bodyregion of a floating state, the layout area is smaller than that ofembodiment 56.

Embodiment 58

FIG. 72 is a diagram showing the concept of a DRAM according toembodiment 58 of the present invention. Referring to FIG. 72, the bodyregion of all P channel MOS transistors is set to a floating state inembodiment 58. Therefore, the body regions of all N channel MOStransistors are electrically fixed, and the body regions of all Pchannel MOS transistors are set to a floating state in embodiment 58.Although the breakdown voltage between the source and drain of an Nchannel MOS transistor having a body region of a floating state isgenerally lower than that of a P channel MOS transistor, the breakdownvoltage between the source and drain of an n channel MOS transistor isincreased substantially to the level of that of a P channel MOStransistor since the body region of N channel MOS transistor iselectrically fixed in embodiment 58. It is therefore not necessary toprovide a body fix line in the region of the P channel MOS transistorsince the breakdown voltage between the source and drain of alltransistors is high and the body region of a P channel MOS transistor isnot electrically fixed. Thus, the layout area of the DRAM of FIG. 58 issmaller than that of embodiment 56.

Although ground potential V_(SS) is applied to the body region of some Nchannel MOS transistors in embodiment 58, a negative potential V_(BB)may be applied, instead of potential V_(SS).

Embodiment 59

FIG. 73 is a diagram showing the concept of a DRAM according toembodiment 59 of the present invention. Referring to FIG. 73, the bodyregion of some N channel MOS transistors are set to a floating state.Therefore, the body regions of some N channel MOS transistors are fixed,and the body regions of all P channel MOS transistors are set to afloating state. Because it is not necessary to provide a body fix linein the region of the some N channel MOS transistors, the layout area ofembodiment 59 is smaller than that of embodiment 58.

Embodiment 60

FIG. 74 is a diagram showing the concept of DRAM according to embodiment60 of the present invention. Referring to FIG. 74, all P channel MOStransistors are isolated by a LOCOS. Some N channel MOS transistor areisolated by a LOCOS, and the remaining N channel MOS transistors areisolated by a field shield (FS). Negative potential V_(BB) is applied tothe body region of the N channel MOS transistor isolated by a fieldshield, and ground potential V_(SS) is applied to the body region of theN channel MOS transistor isolated by a LOCOS. Power supply potentialV_(CC) is supplied to the body regions of all P channel MOS transistors.It is to be noted that ground potential V_(SS) may be applied to thebody regions of the N channel MOS transistor isolated by a field shield.

Embodiment 61

FIG. 75 is a diagram showing the concept of a DRAM according toembodiment 61 of the present invention. Referring to FIG. 75, some Pchannel MOS transistors are isolated by a field shield, and theremaining P channel MOS transistors are isolated by a LOCOS. All Nchannel MOS transistors are isolated by a LOCOS. Power supply potentialV_(CC) is applied to the body regions of all P channel MOS transistors.Some N channel MOS transistors have their body regions set to a floatingstate. The remaining N channel MOS transistors have their body regionssupplied with ground potential V_(SS). In embodiment 61, the bodyregions of some N channel MOS transistors are electrically fixed, andthe body regions of all P channel MOS transistors are electricallyfixed.

Embodiment 62

FIG. 76 is a diagram showing a concept of a DRAM according to embodiment62 of the present invention. Referring to FIG. 76, this DRAM includes aplurality of P channel MOS transistors, and a plurality of N channel MOStransistors. Some P channel MOS transistors have a threshold voltageV_(thp) 1, and the remaining P channel MOS transistors have a thresholdvoltage V_(thp) 2. All the N channel MOS transistors have a thresholdvoltage of V_(thn). Therefore, these P channel MOS transistors have twotypes of threshold voltages. The N channel MOS transistors have one typeof threshold voltage. Thus, a channel type MOS transistor of the sameconductivity type may have two types of threshold voltages.

In order to provide two types of threshold voltages in a transistor, twotypes of potentials are applied to the body regions of thosetransistors. This is because difference in the potential applied to abody region will result in different threshold voltages due to a bodyeffect.

Alternatively, impurities differing in concentration may be doped intothe n type body region of P channel MOS transistor 3 as shown in FIG.77. Regions of different impurity concentration will be formed in theproximity of the surface of body region 3, whereby the two P channel MOStransistors have different threshold voltages.

Alternatively, a gate electrode 4 differing in material may be formed ofthe transistors. In this case, the threshold voltages of thesetransistors will differ from each other according to the work functionspecific to these materials.

Alternatively, SOI active layer 9 may be partially etched to result in athick portion and a thin portion with transistors formed thereon. Thetransistor formed on the thin SOI active layer 9 approximates theso-called fully depleted transistor. The threshold voltage of a fullydepleted transistor is generally smaller than that of a partiallydepleted transistor. Therefore, the transistor formed on the thin SOIactive layer 9 has a threshold voltage smaller than that of thetransistor formed on the thick SOI active layer 9.

The threshold voltage of a transistor may be changed by varying the filmthickness or the material of the gate insulation film.

Although the P channel MOS transistors have two types of thresholdvoltages in embodiment 62, the N channel MOS transistors may have twotypes of threshold voltages. Furthermore, the transistor may have morethan two types of threshold voltages.

As described above, the DRAM may operate more stably if many transistorsin the DRAM formed on a SOI substrate have more than one type ofthreshold voltages.

Embodiment 63

FIG. 78 is a diagram showing the concept of a DRAM according toembodiment 63 of the present invention. Referring to FIG. 78, the bodyregion of the transistor of a short gate length is electrically fixed,and the body region of the transistor of a long gate length iselectrically floating. In general, the breakdown voltage between thesource and drain of a transistor of a long gate length is higher thanthat of a transistor of a short gate length. Therefore, when the bodyregion of the transistor of the short gate length is fixed, thethreshold voltage thereof becomes approximately equal to that of atransistor having a body region of a floating state and a long gatelength. In this case, there is hardly no increase in the layout areasince it is not necessary to arrange a body fix line in the region ofthe transistor of the long gate length.

Embodiment 64

FIG. 79 is a sectional view showing a structure of the sense amplifierof a DRAM according to embodiment 64 of the present invention. Referringto FIG. 79, SOI active layer 9 is etched in a mesa-manner, and the LOCOSoxide film 5 of FIG. 6 is not formed.

FIGS. 80 and 81 are sectional views of a memory cell of a DRAM shown inFIG. 79. This memory cell has a planar structure. SOI active layer 9 isetched in a mesa-manner differing from that of FIGS. 60 and 61.

Such a structure is manufactured by steps set forth in the following.

After source/drain region 44, contact region 31, and the like are formedin SOI active layer 9, SOI active layer 9 is completely etched exceptfor the element active region. As a result, the element active region isformed in a mesa-manner. Then, a gate oxide film is formed so as tocover the mesa element active region. A gate electrode 4 is formed onthis oxide film.

In the case of the LOCOS isolation shown in FIG. 6, there is a problemthat boron implanted into P type body region 3 is absorbed into LOCOSoxide film 5 during formation of LOCOS oxide film 5 carried out bythermal oxidation of SOI active layer 9. This absorption of boron frombody region 3 into LOCOS oxide film 5 causes reduction in the impurityconcentration of edge portion 3 a of body region 3. As a result, aparasitic MOS transistor of a low threshold value is formed in this edgeportion 3 a. Therefore, the so-called hump phenomenon appears in thedrain current-gate voltage characteristics of transistor Qn1. This humpphenomenon is considered to be caused also by a bird's beak specific toLOCOS isolation providing stress to the thin SOI active layer 9.

In contrast, the impurity concentration of the edge portion in bodyregion 3 does not decrease in the case of mesa isolation shown in FIGS.79-81. This is because SOI active layer 9 is not subjected to thermaloxidation, and because body region 3 is covered by a gate oxide film andgate electrode 4. Furthermore, stress will not be generated in the edgeportion of body region 3 because an oxide film, a nitride film, or thelike is deposited as interlayer insulation film 33 by CVD. Thus, a humpphenomenon will not appear in the drain current-gate voltagecharacteristics of this transistor. This transistor can operate in afurther stable manner.

Embodiment 65

FIG. 82 is a sectional view of a memory cell of a DRAM according toembodiment 65 of the present invention. Referring to FIG. 82, thismemory cell has a stack structure. SOI active layer 9 is etched in amesa-manner. It is appreciated from embodiments 64 and 65 that mesaisolation may be employed instead of LOCOS isolation.

Embodiment 66

FIG. 83 is a diagram showing the concept of a DRAM partially accordingto embodiment 66 of the present invention. Although the above-describedembodiment does not mention the potential of silicon substrate 7, it ispreferable to supply a predetermined substrate potential V_(BB) tosilicon substrate 7 as shown in FIG. 83. This substrate potential V_(BB)is generated by a substrate potential generator 100.

In SOI substrate 6, silicon substrate 7 is isolated from SOI activelayer 9 by buried oxide layer 8. However SOI active layer 9 is connectedto silicon substrate 7 via a parasitic capacitance. Therefore, whensilicon substrate 7 is electrically floating, the potential of bodyregion 3 is apt to become unstable in accordance with the potentialvariation of silicon substrate 7. In embodiment 66, the potential ofsilicon substrate 7 does not change since a predetermined substratepotential V_(BB) is provided to silicon substrate 7 so that it iselectrically fixed. Thus, a semiconductor element such as a transistorformed on SOI substrate 6 operates stably.

Embodiment 67

FIG. 84 is a diagram showing the concept of a DRAM according toembodiment 67 of the present invention. The DRAM of embodiment 67differs from that of FIG. 83 in that silicon substrate 7 is connected toground node 51. Because ground potential V_(SS) is supplied to siliconsubstrate 7, silicon substrate 7 is electrically fixed. Therefore, asemiconductor element such as a transistor formed on SOI substrate 6operates stably. It is appreciated from embodiment 67 that the potentialis not particularly limited in the present invention. Not only substratepotential V_(BB), but also ground potential V_(SS) may be applied tosilicon substrate 7.

Embodiment 68

FIG. 85 is a perspective view of a structure to supply substratepotential V_(BB) to silicon substrate 7 as shown in FIG. 83. Referringto FIG. 85, a substrate potential generator 100 is formed on SOIsubstrate 6. A bonding pad 102 is formed on SOI substrate 6. Substratepotential V_(BB) is provided to bonding pad 102 from substrate potentialgenerator 100.

SOI substrate 6 is provided on a die pad (conductor plate) 106 disposedin the package. Bonding pad 102 is connected to die pad 106 via a wire104. Since the back face of SOI substrate 6 is in contact with die pad106, substrate potential V_(BB) generated by substrate potentialgenerator 100 is provided to silicon substrate 7 via bonding pad 102,wire 104, and die pad 106. Thus, silicon substrate 7 is electricallyfixed.

Embodiment 69

FIG. 86 is a perspective view showing a specific structure to provideground potential V_(SS) to silicon substrate 7 as shown in FIG. 84.Bonding pad 102 of FIG. 86 serves to supply ground potential V_(SS) tothe circuit formed on SOI substrate 6. Bonding pad 102 is connected tolead frame 110 to which ground potential V_(SS) is supplied via wire104.

In the present embodiment, die pad 106 is connected to lead frame 110via wire 104. Therefore, ground potential V_(SS) is supplied to siliconsubstrate 7 via lead frame 110, wire 104 and die pad 106. Therefore,silicon substrate 7 is electrically fixed.

Embodiment 70

FIG. 87 is a perspective view showing another example for supplyingground potential V_(SS) to silicon substrate 7. Referring to FIG. 87,SOI substrate 6 is provided on a die pad 112 of a L shape. Bonding pad102 serves to provide ground potential V_(SS) to the circuit on SOIsubstrate 6. Bonding pad 102 is connected to die pad 112 via wire 104.Therefore, ground potential V_(SS) is supplied to bonding pad 102 viadie pad 112, wire 104, and also to silicon substrate 7 via die pad 112.Therefore, silicon substrate 7 is electrically fixed.

Embodiment 71

FIG. 88 is a sectional view showing another example of providingsubstrate potential V_(BB) to silicon substrate 7. Referring to FIG. 88,a contact trench 118 is formed in SOI substrate 6. Trench 118 goesthrough buried oxide layer 8 to silicon substrate 7. A contact hole CHis formed on trench 118. A substrate fix line 114 is formed on contacthole CH. Substrate fix line 114 is connected to silicon substrate 7 viacontact hole CH.

In embodiment 72, substrate potential V_(BB) generated by substratepotential generator 100 is supplied to substrate fix line 114.Therefore, substrate potential V_(BB) is supplied to silicon substrate 7via substrate fix line 114. Therefore, silicon substrate 7 iselectrically fixed.

Embodiment 72

FIG. 89 is a sectional view showing another example of providingsubstrate potential V_(BB) or ground potential V_(SS) to siliconsubstrate 7 of FIGS. 83 or 84. Substrate fix line 114 is connected tobonding pad 102. Bonding pad 102 is formed on SOI substrate 6 as shownin FIG. 86 or 87. Ground potential V_(SS) or substrate potential V_(BB)is supplied to bonding pad 102. Therefore, potential V_(SS) or V_(BB) ofbonding pad 102 is supplied to silicon substrate 7 via substrate fixline 114. Therefore, silicon substrate 7 is electrically fixed.

Embodiment 73

FIG. 90 is a circuit diagram showing a structure of a memory cell, asense amplifier, and an input/output circuit of a DRAM according toembodiment 74 of the present invention. Referring to FIG. 90, a boostedsense ground potential generator 120 is provided. Boosted sense groundpotential V_(BSG) generated by generator 120 is applied to the sourceelectrodes of transistors Qs1 and Qs2.

FIG. 91 is a timing chart showing an operation of this DRAM. As shown inFIG. 91(j), the potential of one bit line only falls to the level ofboosted sense ground potential V_(BSG). This potential V_(BSG) is higherthan ground potential V_(SS) by ΔV.

Although the gate potential of transfer gate Qm in a deselect memorycell is 0V (a L level), the source potential of transfer gate Qm fallsonly to the level of boosted sense ground potential V_(BSG). Therefore,the source potential thereof is higher than the gate potential by ΔV.Therefore, transfer gate Qm attains a more-heavy non-conducting state incomparison with those of the above-described embodiment. In other words,the threshold value of this transfer gate Qm is substantially higher.Therefore, in a de-select memory cell 27, the disturb type subthresholdleakage current is significantly suppressed.

According to such a boosted sense ground method, the threshold value oftransfer gate Qm can be substantially increased without dopingimpurities into the body region thereof. Therefore, the carrier mobilitywill not be reduced by doping. The manufacturing process is simplifiedsince such a doping step is not required.

The potential is not limited to that described in the above embodimentswhere a ground potential V_(SS) or a negative potential V_(BB) isapplied to the body region of a N channel MOS transistor, and anypotential lower than a source potential of the N channel MOS transistormay be applied. Furthermore, although the power supply potential V_(CC)is supplied to the body region of a P channel MOS transistor, anypotential higher than a source potential of the P channel MOS transistormay be applied.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A semiconductor memory device comprising aplurality of first and second conductive channel type MOS transistors,wherein said plurality of first and second conductive channel type MOStransistor are formed on an SOI substrate, wherein at least one firstconductive channel type MOS transistor out of said plurality of firstconductive channel type MOS transistors has a first threshold voltage,wherein at least another first conductive channel type MOS transistorout of said plurality of first conductive channel type MOS transistorshas a second threshold voltage differing from said first thresholdvoltage.
 2. A semiconductor memory device comprising a plurality offirst and second conductive channel type MOS transistors, wherein saidfirst and second conductive channel type MOS transistors are formed onan SOI substrate, wherein a second conductivity type body region locatedbetween a first conductivity type source region and a first conductivitytype drain region of at least one of said plurality of first conductivechannel type MOS transistor includes a conductive layer having a firstimpurity concentration at the surface thereof, wherein a secondconductivity type body region located between a first conductivity typesource region and a first conductivity type drain region of at leastanother of said plurality of first conductive channel MOS transistorsincludes a conductive layer having a second impurity concentrationdiffering from said first impurity concentration at the surface thereof.3. A semiconductor memory device comprising a plurality of first andsecond conductive channel type MOS transistors, wherein said pluralityof first and second conductive channel type MOS transistors are formedon an SOI substrate, wherein a second conductivity type body regionlocated between a first conductivity type source region and a firstconductivity type drain region of at least another of said plurality offirst conductive channel type MOS transistors is supplied with a firstpotential, wherein a second conductivity type body region locatedbetween a first conductivity type source region and a first conductivitytype drain region of at least one of said plurality of first conductivechannel type MOS transistors is supplied with a second potentialdifferent from said first potential.
 4. A semiconductor memory devicecomprising a memory cell array including a plurality of first MOStransistors and a peripheral circuit including a plurality of second MOStransistors, wherein said plurality of first and second MOS transistorsare formed on an SOI substrate, wherein each of said plurality of firstMOS transistors has a threshold voltage higher than the thresholdvoltage of each of said plurality of second MOS transistors.